Gossamer Bio, Inc.

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A

Proxy Statement Pursuant to Section 14(a) of the

Securities Exchange Act of 1934

(Amendment No. )

Filed by the Registrantx Filed by a Party other than the Registrant¨

Check the appropriate box:

¨Preliminary Proxy Statement

¨ Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2))

x Definitive Proxy Statement

¨ Definitive Additional Materials

¨ Soliciting Material under §240.14a-12

Gossamer Bio, Inc.

(Name of Registrant as Specified In Its Charter)

(Name of Person(s) Filing Proxy Statement, if other than the Registrant)

Payment of Filing Fee (Check all boxes that apply):

September 16, 2026

Dear Stockholder:

On behalf of the board of directors (the “Board”) of Gossamer Bio, Inc., a Delaware corporation (“we,” “us,” “our,” “Gossamer” or the “Company”), you are cordially invited to attend a Special Meeting of Stockholders (the “Special Meeting”), to be held virtually, via live webcast, on Tuesday, October 27, 2026 at 9:00 a.m. Pacific Time, unless postponed or adjourned to a later date.

Background of the Special Meeting

The Special Meeting is being held to consider proposals arising out of the Company’s private placement of securities (the “Private Placement”) pursuant to that certain securities purchase agreement (the “Purchase Agreement”), dated August 20, 2026, by and among the Company and the purchasers named therein, including certain executive officers and directors of the Company (collectively, the “Purchasers”).

At the initial closing held on August 24, 2026 (the “Initial Closing”), the Company issued pre-funded warrants (the “Initial Closing Pre-Funded Warrants”), exercisable for, (i) prior to obtaining Stockholder Approval (as defined below), up to 25,000 shares of the Company’s Series A-1 Convertible Preferred Stock, par value $0.0001 per share (the “Series A-1 Preferred Stock”), or (ii) following the receipt of Stockholder Approval, approximately 2,233,742 shares of the Company’s Common Stock, par value $0.0001 per share (the “Common Stock”), in each case on the terms set forth in the Initial Closing Pre-Funded Warrants. The purchase price per Initial Closing Pre-Funded Warrant was $11.192 (representing a per share price of Common Stock of $11.20 less the exercise price of $0.008 per Initial Closing Pre-Funded Warrant).

At the second closing (the “Second Closing”), to occur upon the acceptance by the United States Food and Drug Administration (the “FDA”) of filing of the Company’s New Drug Application (“NDA”) for seralutinib for the treatment of patients with pulmonary arterial hypertension (“PAH”) (the “NDA Acceptance Milestone”), subject to the NDA Acceptance Milestone occurring in 2026 and the satisfaction of customary closing conditions, the Company has agreed to issue and sell, and the Purchasers have agreed to purchase, severally and not jointly, (i) pre-funded warrants (the “Second Closing Pre-Funded Warrants”), exercisable for, (A) prior to obtaining Stockholder Approval, up to 125,000 shares of the Company’s Series A-2 Convertible Preferred Stock, par value $0.0001 per share (the “Series A-2 Preferred Stock”), or (B) following the receipt of Stockholder Approval, a number of shares of Common Stock equal to approximately $125 million divided by the purchase price of each Second Closing Pre-Funded Warrant, calculated in the manner described below, in each case on the terms set forth in the Second Closing Pre-Funded Warrants, and (ii) warrants (the “FDA Approval Warrants”), exercisable for, (A) prior to obtaining Stockholder Approval, up to 100,000 shares of the Company’s Series A-3 Convertible Preferred Stock, par value $0.0001 per share (the “Series A-3 Preferred Stock,” and together with the Series A-1 Preferred Stock and Series A-2 Preferred Stock, the “Preferred Stock”), or (B) following the receipt of Stockholder Approval, approximately 6,684,493 shares of Common Stock (based on a per share price of Common Stock of $14.96), or in lieu thereof, pre-funded warrants (the “FDA Approval Warrant Pre-Funded Warrants,” and together with the Initial Closing Pre-Funded Warrants and Second Closing Pre-Funded Warrants, the “Pre-Funded Warrants”), in each case on the terms set forth in the FDA Approval Warrants. The purchase price per Second Closing Pre-Funded Warrant will be equal to the lesser of (x) $11.192 per Pre-Funded Warrant and (y) the volume-weighted average price per share of Common Stock during the five consecutive trading days ending on and including the trading day immediately preceding the date of the Second Closing, less the exercise price of $0.0001 per Second Closing Pre-Funded Warrant. The FDA Approval Warrants are only exercisable following approval by the FDA of the Company’s NDA for seralutinib for the treatment of patients with PAH (the “FDA Approval Milestone”).

The issuance of shares of Common Stock upon exercise of the Pre-Funded Warrants and FDA Approval Warrants and upon conversion of the Preferred Stock is subject to the approval of the Company’s stockholders in accordance with the applicable rules and regulations of the Nasdaq Global Select Market (“Stockholder Approval”).

Purpose of the Special Meeting

Our Board has called the Special Meeting to consider the following proposals, each of which is more fully described in the accompanying proxy statement:

Board Recommendation

Our Board has determined that each of the proposals is in the best interests of the Company and its stockholders and unanimously recommends that you vote “FOR” each proposal.

Reverse Stock Split

As more fully described in the accompanying proxy statement, on September 10, 2026 at 11:59 p.m. Eastern Time (the “Effective Time”), the Company effected a 1-for-80 reverse stock split of the Company’s Common Stock (the “Reverse Stock Split”). As a result of the Reverse Stock Split, every 80 shares of the Company’s Common Stock issued and outstanding immediately prior to the Effective Time were automatically reclassified and combined into 1 share of Common Stock. All shares of the Company’s Common Stock (or securities convertible into or exercisable for shares of the Company’s Common Stock), per-share data and related information included herein and in the accompanying proxy statement have been adjusted to reflect the Reverse Stock Split.

Conclusion

The accompanying proxy statement provides you with information about the proposals. Please read the entire proxy statement carefully. You may obtain additional information about the Company from documents we file with the Securities and Exchange Commission.

It is important that your shares be represented and voted at the Special Meeting. Please submit your proxy as soon as possible even if you plan to attend the Special Meeting. We appreciate your support regarding this matter. Only those holders of record of shares of Common Stock at 4:00 p.m. Eastern Time on September 14, 2026, the record date for the Special Meeting, will be entitled to vote at the Special Meeting (the “Record Date”) or any adjournment or postponement thereof.

Thank you for your ongoing support of, and continued interest in, Gossamer.

Sincerely,

/s/ Faheem Hasnain

Faheem HasnainChairman, President and Chief Executive Officer

NOTICE OF SPECIAL MEETING OF STOCKHOLDERS

To Be Held at 9:00 a.m. Pacific Time on Tuesday, October 27, 2026

Dear Stockholder:

On behalf of the board of directors (the “Board”), of Gossamer Bio, Inc., a Delaware corporation (“we,” “us,” “our,” “Gossamer” or the “Company”), you are cordially invited to attend a Special Meeting of Stockholders (the “Special Meeting”), to be held virtually, via live webcast, on Tuesday, October 27, 2026 at 9:00 a.m. Pacific Time, for the following purposes, as more fully described in the accompanying proxy statement:

1. To approve, in accordance with Nasdaq Listing Rules 5635(c) and 5635(d), the potential issuance of shares of the Company’s Common Stock, par value $0.0001 per share (the “Common Stock”) upon (i) the exercise of the Pre-Funded Warrants (as defined in the accompanying proxy statement) issued or issuable pursuant to the Purchase Agreement, (ii) the exercise of the FDA Approval Warrants (as defined in the accompanying proxy statement) issuable pursuant to the Purchase Agreement, and (iii) the conversion of shares of the Series A-1 Preferred Stock, Series A-2 Preferred Stock and Series A-3 Preferred Stock (each as defined in the accompanying proxy statement), in each case issuable upon exercise of the Pre-Funded Warrants and the FDA Approval Warrants, as applicable, including shares of Common Stock issuable to certain executive officers and directors of the Company, which issuances of Common Stock would, in the aggregate, exceed 20% of the number of shares of Common Stock issued and outstanding immediately prior to the Initial Closing (as defined in the accompanying proxy statement), as described in Proposal No. 1 in the accompanying proxy statement; and

2. To approve one or more adjournments of the Special Meeting, if necessary, to solicit additional proxies if there are not sufficient votes to approve the foregoing proposal at the Special Meeting or any adjournment(s) thereof, as described in Proposal No. 2 in the accompanying proxy statement.

The Board has fixed 4:00 p.m. Eastern Time on September 14, 2026 as the record date (the “Record Date”), for the Special Meeting. Stockholders of record as of the Record Date are entitled to notice of and to vote at the Special Meeting. Further information regarding voting rights and the matters to be voted upon is presented in the accompanying proxy statement.

This Notice of Special Meeting of Stockholders, the proxy statement, and the proxy card are being sent to stockholders beginning on or about September 16, 2026. The proxy card or voting instructions received from your broker, bank or other nominee provide instructions on how to vote. The proxy statement can be accessed directly at the following Internet address: www.proxydocs.com/GOSS. You will be asked to enter the Control Number shown on your proxy card or voting instructions received from your broker, bank or other nominee.

We appreciate your continued support of Gossamer.

By Order of the Board of Directors,

/s/ Faheem Hasnain

Faheem Hasnain

Chairman, President and Chief Executive Officer

San Diego, California

September 16, 2026

Your vote is important. Please vote your shares whether or not you plan to attend the meeting.

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