Gossamer Bio, Inc.

There have been no material changes to the risk factors previously disclosed by us in Part I, Item 1A “Risk Factors” of our Annual Report on Form 10-K for the fiscal year ended December 31, 2025 filed with the SEC on March 17, 2026 and Part II, Item 1A "Risk Factors" of our Quarterly Report on Form 10-Q for the quarter ended June 30, 2026 filed with the SEC on August 13, 2026, except as set forth below:

The results from the PROSERA Phase 3 clinical trial may not be sufficient to support FDA acceptance for filing of our NDA or approval or continued development of seralutinib, which would materially and adversely harm our business.

In February 2026, we announced topline results from the Phase 3 PROSERA clinical trial of seralutinib in PAH, including that the study did not meet its primary endpoint. Following a Pre-NDA Type B meeting with the FDA held in mid-June 2026 and receipt of the official meeting minutes, we plan to proceed with an NDA submission for seralutinib for the treatment of PAH in September 2026. Based on the meeting minutes, the FDA characterized the degree of statistical significance and the magnitude of the treatment effect observed in PROSERA as review issues rather than filing issues. In September 2026 we submitted an NDA submission for seralutinib for the treatment of PAH based in part on our views following our meeting with the FDA and the official minutes therefrom. However, later feedback from, or developments with, the FDA may be inconsistent with our prior meeting or our views from such meeting. The FDA may determine that our NDA does not qualify for filing and may not accept the NDA for filing. Even if the FDA accepts the NDA for filing, the FDA's ultimate determination on approvability will be made upon review of the complete NDA, and there can be no assurance that the FDA will ultimately approve the seralutinib NDA. In general, the FDA has substantial discretion in the approval process and may decide that the totality of our datasets, including the Phase 3 PROSERA and Phase 2 TORREY studies, have not demonstrated a favorable overall benefit-risk assessment or may otherwise determine are insufficient for approval and require additional clinical trials or other studies, especially given that the PROSERA study did not meet its primary endpoint. Any such decision or feedback from the FDA would result in additional development costs and could significantly delay the potential for regulatory approval, or even if we are approved, a more narrow or limited labeled indication. We also may be unable to identify a viable development path towards approval for seralutinib, based on FDA feedback, our internal analysis of the data and market opportunity, or other factors. Even if we do identify a path to approval for seralutinib, we may require substantial additional capital and other resources to pursue such a path, and may be unable to raise such capital in the amounts needed or on attractive terms. There is also no assurance that any future trials or studies we may need to conduct will be successful.

We may also pursue business development arrangements or other strategic collaborations; however, we may be unable to secure such arrangements on a timely basis or at all. If we are unable to develop or seek marketing approval for seralutinib or secure other strategic arrangements with third parties, or if we experience delays as a result of any of the above factors or otherwise, our business would be significantly harmed.

We may not receive some or all of the additional proceeds contemplated by the Private Placement, which could adversely affect our liquidity and our ability to fund our operations.

On August 20, 2026, we entered into the Purchase Agreement providing for the Private Placement in two closings. Subject to the NDA Acceptance Milestone occurring in 2026 and the satisfaction of customary closing conditions, the purchasers have agreed to purchase the Second Closing Pre-Funded Warrants at the Second Closing for additional gross proceeds of approximately $125.0 million. If the Second Closing does not occur on or before December 31, 2026, the obligations relating to the Second Closing Pre-Funded Warrants and the FDA Approval Warrants will automatically terminate under the Purchase Agreement. There can be no assurance that the NDA Acceptance Milestone will be achieved within the required timeframe, or at all, or that the other conditions to the Second Closing will be satisfied or waived. If the NDA Acceptance Milestone is not achieved in 2026, or the Second Closing does not otherwise occur on or before December 31, 2026, we will not receive the proceeds from the Second Closing. In addition, the FDA Approval Warrants, which purchasers would receive at the Second Closing, become exercisable only after the FDA Approval Milestone. They represent potential additional gross proceeds of up to approximately $100.0 million if exercised in full. If the Second Closing does not occur, the FDA Approval Warrants will not be issued. Even if the FDA Approval Warrants are issued, there can be no assurance that the FDA Approval Milestone will be achieved or that holders will exercise the FDA Approval Warrants for cash, in whole or in part.

If the Second Closing does not occur, our liquidity and ability to fund our operations would be materially and adversely affected, and we would be required to raise additional capital to support any additional development required by the FDA prior to seeking regulatory approval of any future NDA submission. In addition, pursuant to the Purchase Agreement governing the Private Placement, we are subject to restrictions on our ability to issue additional equity securities, subject to certain exceptions, which could limit our ability to raise additional capital through equity financings while such restrictions remain in effect. We would expect to finance our cash needs through equity offerings, debt financings or other capital sources, including potentially collaborations, licenses and other similar arrangements. However, we may be unable to raise additional funds or enter into such other arrangements when needed on favorable terms or at all. Our failure to raise capital or enter into such other arrangements when needed could have a negative impact on our financial condition and on our ability to pursue our business plans and strategies. If we are unable to raise additional capital when needed, we could be forced to delay, limit, reduce or terminate seralutinib development or grant rights to develop and market seralutinib even if we would otherwise prefer to develop and market seralutinib ourselves. Any of these outcomes could materially adversely affect our business, financial condition and results of operations.