GP-Act III Acquisition Corp.

The extraordinary general meeting (the “Extraordinary General Meeting”) of GP-Act III Acquisition Corp. (“we,” “us,” “our” or the “Company”) will be held at 9:00 a.m. Eastern Time on November 12, 2026 at the offices of Skadden, Arps, Slate, Meagher & Flom LLP, located at Avenida Brigadeiro Faria Lima, 3311, 7 th Floor, 04538-133, São Paulo, São Paulo, Brazil, or at such other time, on such other date and at such other place to which the meeting may be postponed or adjourned, or to attend virtually via the Internet. You will be able to attend the Extraordinary General Meeting online, vote, view the list of shareholders entitled to vote at the Extraordinary General Meeting and submit your questions during the Extraordinary General Meeting by visiting https://www.cstproxy.com/gp-act3/egm2026 or by phone dialing within the U.S. and Canada 1 800-450-7155 (toll-free) or outside of the U.S. and Canada +1 857-999-9155 (standard rates apply) and entering the conference identification number 3650298#. The sole purpose of the Extraordinary General Meeting is to consider and vote upon the following proposals:

• a proposal to amend the Company’s amended and restated memorandum and articles of association (the “Articles”) pursuant to an amendment to the Articles in the form set forth in Annex A to this Proxy Statement (the “Extension Amendment” and, such proposal, the “Extension Amendment Proposal”) to extend the date by which the Company must (1) consummate a merger, amalgamation, share exchange, asset acquisition, share purchase, reorganization or similar business combination with one or more businesses (a “business combination”), (2) cease its operations except for the purpose of winding up if it fails to complete such business combination, and (3) redeem all of the Company’s Class A ordinary shares included as part of the units sold in the Company’s initial public offering that was consummated on May 13, 2024 (the “IPO”), from November 13, 2026 to May 8, 2027 (the “Extension” and, such date, the “Extended Date”);

• a proposal to amend the Investment Management Trust Agreement (the “Trust Agreement”), dated May 8, 2024, as first amended on May 12, 2026, by and between the Company and Continental Stock Transfer & Trust Company, as trustee (“Continental”), pursuant to an amendment to the Trust Agreement in the form set forth in Annex B to this Proxy Statement, to extend the date on which Continental must liquidate the Trust Account (the “Trust Account”) established in connection with the IPO if the Company has not completed its initial business combination, from November 13, 2026 to May 8, 2027 (the “Trust Amendment” and, such proposal, the “Trust Amendment Proposal”); and

• a proposal to approve the adjournment of the Extraordinary General Meeting to a later date or dates, if necessary, (i) to permit further solicitation and vote of proxies in the event that there are insufficient votes for, or otherwise in connection with, the approval of the Extension Amendment Proposal or the Trust Amendment Proposal or (ii) where the Board has determined it is otherwise necessary (the “Adjournment Proposal”). The Adjournment Proposal will only be presented at the Extraordinary General Meeting if there are not sufficient votes to approve the Extension Amendment Proposal and the Trust Amendment Proposal.

The purpose of the Extension Amendment and the Trust Amendment is to allow us more time to enter into and complete a potential business combination we are currently negotiating with a prospective target entity (the “Target” and the “Potential Business Combination”) in case such additional time is needed. On July 10, 2026, we entered into a non-binding term sheet with the Target with respect to the Potential Business Combination. The Target is an aerospace and logistics technology company developing autonomous cargo aircraft and is currently in the pre-commercialization stage of development.

Since executing the term sheet, we and the Target have made substantial progress toward negotiating and finalizing a definitive business combination agreement (the “Business Combination Agreement”). We have engaged a renowned international financial advisor, and the parties have engaged legal counsel in the United States and several other relevant jurisdictions. The parties have exchanged multiple drafts of the Business Combination Agreement and related transaction documents, and negotiations are at an advanced stage. The parties currently contemplate that the Potential Business Combination will be structured as a share exchange pursuant to which the Target equity will be transferred to us in exchange for our shares at a determined exchange ratio. All parties are working toward finalizing and executing the Business Combination Agreement.

Except for certain customary provisions, the term sheet is non-binding, and there can be no assurance that we will enter into the Business Combination Agreement on the terms contemplated by the term sheet or at all, or, if we do, that the Potential Business Combination will be consummated.

The Articles provide that we have until November 13, 2026 to complete our initial business combination. The Potential Business Combination involves a number of complex legal, regulatory, tax and structuring considerations across multiple jurisdictions, which have contributed to the time required to negotiate and finalize the transaction documentation. While the parties have made substantial progress in addressing these matters, with negotiations now in their advanced stages, our board believes that there may not be sufficient time before November 13, 2026 to hold an extraordinary general meeting at which to conduct a vote for the shareholder approvals required in connection with the Potential Business Combination and consummate the closing of the Potential Business Combination. Accordingly, our board believes that in order for us to finalize the negotiation of the Potential Business Combination, our shareholders to then evaluate the Potential Business Combination and for us to be able to potentially consummate the Potential Business Combination, we will need to obtain the Extension. However, if the Potential Business Combination negotiations are terminated before the Extraordinary General Meeting, we will not hold the Extraordinary General Meeting.

Approval of the Extension Amendment Proposal and the Trust Amendment Proposal are both a condition to the implementation of the Extension.

In connection with the Extension Amendment Proposal, public shareholders (as defined below) may elect to redeem their public shares(as defined below) for a per-share price, payable in cash, equal to the aggregate amount then on deposit in the Trust Account, including interest earned, divided by the number of then outstanding Class A ordinary shares included as part of the units sold in the IPO (whether purchased in the IPO or thereafter in the open market, but excluding any converted Class B ordinary shares, the “public shares”), and which election we refer to as the “Election.” An Election can be made regardless of whether such public shareholders vote “FOR” or “AGAINST” the Extension Amendment Proposal and the Trust Amendment Proposal and an Election can also be made by public shareholders who do not vote, or do not instruct their broker or bank how to vote, at the Extraordinary General Meeting. Holders of public shares (the “public shareholders”) may make an Election regardless of whether such public shareholders were holders as of the record date. Public shareholders who do not make the Election would be entitled to have their shares redeemed for cash if we have not completed our initial business combination by the Extended Date. In addition, regardless of whether public shareholders vote “FOR” or “AGAINST” the Extension Amendment Proposal and the Trust Amendment Proposal, or do not vote, or do not instruct their broker or bank how to vote, at the Extraordinary General Meeting, if the Extension is implemented and a public shareholder does not make an Election, they will retain the right to vote on any proposed initial business combination in the future, including the Potential Business Combination, if applicable, and the right to redeem their public shares at a per-share price, payable in cash, equal to the aggregate amount then on deposit in the Trust Account as of two business days prior to the consummation of such initial business combination, including interest (which interest shall be net of taxes payable), divided by the number of then outstanding public shares, in the event a proposed business combination, including the Potential Business Combination, if applicable, is completed. We are not asking you to vote on any proposed business combination, including the Potential Business Combination, at this time. If we enter into the Potential Business Combination, we intend to file (i) promptly thereafter a current report on Form 8-K with information about the Potential Business Combination, and (ii) in due course a separate proxy statement/prospectus pursuant to which we will seek approval of the Potential Business Combination, among other things, at a separate extraordinary general meeting. If the Extension is not approved, we may not be able to enter into, nor consummate, the Potential Business Combination. We urge you to vote at the Extraordinary General Meeting regarding the Extension. In addition, if you elect to redeem your shares at this time in connection with the Extension, sufficient cash amounts may not remain in the Trust Account to permit the Company to satisfy a related closing condition that

may be set forth in the transaction documents of the Potential Business Combination, if any. As a result, while the Target would have the right to waive any related closing condition in certain circumstances, the Potential Business Combination may not be consummated if there is not a sufficient amount of cash in the Trust Account as a result of redemptions of our shares in connection with the Extension, even if our shareholders vote to approve the Extension.

On May 1, 2026, GP-Act III Sponsor LLC (“Sponsor HoldCo”) and our three independent directors, as the holders of all of our issued and outstanding Class B ordinary shares (which we refer to herein also as the “founder shares”), converted all 7,187,500 founder shares into Class A ordinary shares on a one-for-one basis. As a result, no Class B ordinary shares remain outstanding. The Class A ordinary shares issued upon the conversion remain subject to all of the terms and conditions that applied to the founder shares under the Letter Agreement, dated May 8, 2024, among us, Sponsor HoldCo and our officers and directors, including the voting agreement, the transfer restrictions and the waiver of any right to monies held in the Trust Account. These shares are not public shares and are not entitled to redemption rights. As used herein, the “ordinary shares” or “shares” means our Class A ordinary shares, consisting of the public shares and the converted founder shares.

As previously reported in the Company’s current report on Form 8-K filed with the SEC on May 14, 2026, on May 12, 2026, in connection with its Extraordinary General Meeting held on May 12, 2026 (the “First Extension Extraordinary General Meeting”), the Company’s shareholders approved: (1) a special resolution to amend the Amended and Restated Memorandum and Articles of Association of the Company to extend the date by which the Company must (a) consummate a business combination, (b) cease its operations except for the purpose of winding up if it fails to complete such business combination, and (c) redeem all of the Company’s Class A ordinary shares included as part of the units sold in the IPO, from May 13, 2026 to November 13, 2026; and (2) the proposal to extend the date on which the Trustee must liquidate the trust account established in connection with the IPO if the Company has not completed its initial business combination, from May 13, 2026 to November 13, 2026. In connection with the First Extension Extraordinary General Meeting and the relevant proposals, holders of 19,776,272 Class A ordinary shares of the Company properly exercised their right to redeem their shares for cash at a redemption price of approximately $10.89 per share, for an aggregate redemption amount of approximately $215.4 million, leaving approximately $97.8 million in the Trust Account.

The withdrawal of funds from the Trust Account in connection with the Election will reduce the amount held in the Trust Account following the Election, and the amount remaining in the Trust Account may be only a small fraction of the $99,392,076.68 that was in the Trust Account as of October 26, 2026. In such event, we may need to obtain additional funds to complete the Potential Business Combination or any other initial business combination, and there can be no assurance that such funds will be available on terms acceptable or at all.

If the Extension Amendment Proposal or the Trust Amendment Proposal is not approved and we do not consummate our initial business combination by November 13, 2026, as contemplated by and in accordance with our Articles, we will: (1) cease all operations except for the purpose of winding up; (2) as promptly as reasonably possible but not more than 10 business days thereafter, redeem the public shares, at a per-share price, payable in cash, equal to the aggregate amount then on deposit in the Trust Account, including interest (less up to $100,000 of interest to pay dissolution expenses and which interest shall be net of taxes payable), divided by the number of then issued and outstanding public shares, which redemption will completely extinguish public shareholders’ rights as shareholders (including the right to receive further liquidation distributions, if any); and (3) as promptly as reasonably possible following such redemption, subject to the approval of our remaining shareholders and our board, dissolve and liquidate, subject in each case to our obligations under Cayman Islands law to provide for claims of creditors and the requirements of other applicable law.

There will be no redemption rights or liquidating distributions with respect to our warrants, which will expire worthless in the event of our winding up. In the event of a liquidation, Sponsor HoldCo and certain other initial shareholders (including Andrew Fleiss, Alexandre Ruberti and Sergio Pedreiro) (the “other initial shareholders”), as holders of the Class A ordinary shares issued upon conversion of our Class B ordinary shares (which we refer to herein as the “founder shares”) will not be entitled to receive any monies held in the Trust Account in respect of those shares.

Based upon the amount in the Trust Account as of October 26, 2026, which was $99,392,076.68, we anticipate that the per-share price at which public shares will be redeemed from cash held in the Trust Account will be approximately $11.07 at the time of the Extraordinary General Meeting. The closing price of the public shares on the Nasdaq Global Market (the “Nasdaq”) on October 22, 2026, the most recent practicable closing price prior to the

mailing of this Proxy Statement, was $11.08. We cannot assure shareholders that they will be able to sell their shares in the open market, even if the market price per share is higher than the redemption price stated above, as there may not be sufficient liquidity in our securities when such shareholders wish to sell their shares.

If the Extension Amendment Proposal and the Trust Amendment Proposal are approved, the approval of the Trust Amendment Proposal will constitute consent for us to (1) remove from the Trust Account an amount (the “Withdrawal Amount”) equal to the number of public shares properly redeemed multiplied by the per-share price, equal to the aggregate amount then on deposit in the Trust Account, including interest earned, divided by the number of then outstanding public shares and (2) deliver to the holders of such redeemed public shares their pro rata portion of the Withdrawal Amount. The remainder of such funds will remain in the Trust Account and will be available for use by us in connection with consummating an initial business combination on or before the Extended Date. Holders of public shares who do not redeem their public shares now will retain their redemption rights and their ability to vote on any initial business combination, including the Potential Business Combination, if applicable, through the Extended Date if the Extension Amendment Proposal and the Trust Amendment Proposal are approved.

Under the Trust Amendment Proposal, we will amend the Trust Agreement to extend the date on which Continental must liquidate the Trust Account to the Extended Date.

Our board has fixed the close of business on October 14, 2026 as the record date for determining our shareholders entitled to receive notice of and vote at the Extraordinary General Meeting and any adjournment thereof. Only holders of record of the ordinary shares on that date are entitled to have their votes counted at the Extraordinary General Meeting or any adjournment thereof.

On the record date of the Extraordinary General Meeting, there were 16,161,228 ordinary shares outstanding, consisting entirely of Class A ordinary shares, including 7,187,500 Class A ordinary shares issued upon conversion of our Class B ordinary shares. On May 1, 2026, Sponsor HoldCo and our other initial shareholders, who together held all of the Class B ordinary shares, converted all 7,187,500 Class B ordinary shares into Class A ordinary shares on a one-for-one basis. As a result, 7,187,500 of our Class B ordinary shares were cancelled, and 7,187,500 Class A ordinary shares were issued to Sponsor HoldCo and our other initial shareholders. Following this event, there are no longer any Class B ordinary shares outstanding. The terms of the Letter Agreement that applied to the founder shares, including the voting agreement, transfer restrictions and the waiver of any right to monies held in the trust account, continue to apply to the Class A ordinary shares issued upon the conversion. These shares carry voting rights in connection with the Extension Amendment Proposal, the Trust Amendment Proposal and the Adjournment Proposal. Our Sponsor HoldCo and our independent directors, who hold all of these shares, have informed us that they intend to vote them in favor of the Extension Amendment Proposal, the Trust Amendment Proposal and the Adjournment Proposal.

This Proxy Statement contains important information about the Extraordinary General Meeting and the proposals. Please read it carefully and vote your shares.

We will pay for the entire cost of soliciting proxies. We have engaged Sodali & Co. (“Sodali”), to assist in the solicitation of proxies for the Extraordinary General Meeting. We have agreed to pay Sodali a fee of $20,000. We will also reimburse Sodali for reasonable out-of-pocket expenses and will indemnify Sodali and its affiliates against certain claims, liabilities, losses, damages and expenses. In addition to these mailed proxy materials, our directors and officers may also solicit proxies in person, by telephone or by other means of communication. These parties will not be paid any additional compensation for soliciting proxies. We may also reimburse brokerage firms, banks and other agents for the cost of forwarding proxy materials to beneficial owners.

This Proxy Statement is dated October 26, 2026 and is first being mailed to shareholders on or about October 26, 2026.