Other Events. On September 23, 2026, Gossamer Bio, Inc. (the “Company”) announced that it submitted a New Drug Application (“NDA”) to the U.S. Food and Drug Administration (“FDA”) in September 2026 for seralutinib for the treatment of pulmonary arterial hypertension (“PAH”). This is the Company’s first NDA submission. The NDA is supported by the Company’s Phase 3 PROSERA Study as one adequate and well-controlled clinical investigation, together with confirmatory evidence from the Company’s Ph…

of the Current Report on Form 8-K filed by the Company with the Securities and Exchange Commission on August 21, 2026 is incorporated herein by reference. On September 16, 2026, the Company issued a press release announcing Mr. Ciongoli’s appointment to the Board. A copy of the press release is attached hereto as Exhibit 99.1 and incorporated herein by this reference.

Director — Gregory A. Ciongoli: The filing discloses the appointment of a new independent director to the board, which is a routine governance event and not a departure of a senior executive.

Material Modification to Rights of Security Holders. To the extent required by

Other Events. On September 11, 2026, the Company issued a press release announcing the effectiveness of the Reverse Stock Split and Authorized Share Reduction. A copy of the press release is attached hereto as Exhibit 99.1 and is incorporated herein by reference. * * * Note Regarding Forward-Looking Statements. The Company cautions you that statements contained in this report regarding matters that are not historical facts are forward-looking statements. These statements are based on the Comp…

Other Events. As previously disclosed, on July 14, 2026, the stockholders of Gossamer Bio, Inc. (the “Company”) approved thirty alternate amendments to the Company’s Amended and Restated Certificate of Incorporation to effect a reverse stock split (the “Reverse Stock Split”) of the Company’s common stock, par value $0.0001 per share (the “Common Stock”), at one of thirty reverse stock split ratios, with the exact ratio to be determined by the board of directors of the Company (the “Board”), a…

of the Current Report on Form 8-K filed by the Company with the Securities and Exchange Commission on August 21, 2026 is incorporated herein by reference, and such description is qualified in its entirety by reference to the Series A-1 Certificate of Designation, which is filed hereto as Exhibit 3.1.

Entry into a Material Definitive Agreement Securities Purchase Agreement and Private Placement On August 20, 2026, Gossamer Bio, Inc. (the “Company”) entered into a securities purchase agreement (the “Purchase Agreement”) with the purchasers named therein, including certain executive officers and directors of the Company (the “Purchasers”), for the private placement (the “Private Placement”) of securities in two closings. At the initial closing (the “Initial Closing”), the Company has agreed…

Unregistered Sales of Equity Securities. The disclosures set forth in

Results of Operations and Financial Condition. On August 13, 2026, Gossamer Bio, Inc. (the "Company") issued a press release reporting its financial results for the quarter ended June 30, 2026. The full text of the press release is attached as Exhibit 99.1 to this Current Report on Form 8-K. In accordance with General Instruction B.2 of Form 8-K, the information contained or incorporated herein, including the press release attached as Exhibit 99.1, shall not be deemed “filed” for purposes of…

Termination of a Material Definitive Agreement. The disclosure set forth in

Other Events. On July 27, 2026, the Company announced that following a Pre-NDA Type B meeting with the U.S. Food and Drug Administration (“FDA”) held in mid-June and receipt of the official meeting minutes, the Company is moving forward with a planned new drug application (“NDA”) submission for seralutinib for the treatment of patients with PAH in September 2026. Based on the meeting minutes, the FDA characterized the degree of statistical significance and the magnitude of the treatment effec…

Results of Operations and Financial Condition. The Company estimates that its cash, cash equivalents and marketable securities were approximately $57.0 million as of June 30, 2026. This amount is unaudited and preliminary and is subject to completion of financial closing procedures. As a result, this amount may differ from the amount that will be reflected in the Company’s financial statements as of and for the quarter ended June 30, 2026. The information in this

Entry Into a Material Definitive Agreement. Rights Reacquisition Agreement On July 23, 2026, Gossamer Bio, Inc. (the “Company” or “Gossamer”), Gossamer Bio USA, Inc. (formerly GB002, Inc.) and Gossamer Bio 002 Ltd., on the one hand, and Chiesi Farmaceutici S.p.A. and Chiesi USA, Inc. (together, “Chiesi”), on the other hand, entered into a Rights Reacquisition Agreement (the “Rights Reacquisition Agreement”), under which Gossamer and Chiesi have agreed (a) to terminate that certain Collaborati…

The filing describes a stockholder approval of an amended and restated incentive award plan, which is not directly related to management changes.

Regulation FD Disclosure. On June 17, 2026, Gossamer Bio, Inc. (“Gossamer” or the “Company”) issued a press release announcing the final tender results for the Company’s previously announced exchange offer (the “Exchange Offer”) to exchange any and all of its 5.00% Convertible Senior Notes due 2027 (the “Existing Convertible Notes”) for a pro rata portion of (i) up to $72.0 million in aggregate principal amount of its new 7.50% Convertible Senior Secured First Lien Notes due 2030, (ii) up to…

Entry into a Material Definitive Agreement. New Convertible Notes Indenture The New Convertible Notes delivered in exchange for the Early Tendered Notes were, and any New Convertible Notes delivered in exchange for remaining Existing Convertible Notes that are validly tendered in the Exchange Offer following the Extended Early Tender Date and at or prior to the Expiration Deadline will be, issued pursuant to an indenture, dated as of June 4, 2026 (the “New Convertible Notes Indenture”), by an…

Termination of a Material Definitive Agreement. Cancellation of Existing Convertible Notes Tendered in Exchange Offer On the Early Settlement Date, the Company caused the Early Tendered Notes accepted for exchange to be delivered to the Existing Convertible Notes Trustee for cancellation. The Early Tendered Notes represented 90.526% of the previously outstanding Existing Convertible Notes. The information set forth in the Explanatory Note and in

Unregistered Sales of Equity Securities. In connection with the Exchange Offer, on the Early Settlement Date, in reliance upon the exemption from the registration requirements of the Securities Act provided by Section 4(a)(2) thereof and Rule 506 of Regulation D promulgated thereunder, the Company issued $65,174,000 in aggregate principal amount of New Convertible Notes, 254,150,441 shares of its Common Stock, 33,402,727 Prefunded Warrants and 135,789,000 Purchase Warrants to eligible holders…

Creation of a Direct Financial Obligation or an Obligation under an Off-Balance Sheet Arrangement. The information set forth in the Explanatory Note and in

Regulation FD Disclosure. On June 3, 2026, the Company issued a press release announcing the early tender results and its election to accept for exchange the Early Tendered Notes in the Exchange Offer and Consent Solicitation. The full text of the press release is furnished herewith as Exhibit 99.1 and incorporated herein by reference. In accordance with General Instruction B.2 of Form 8-K, the information contained or incorporated herein, including the press release attached as Exhibit 99.1,…

Other Events On June 2, 2026, the Company issued a press release announcing the extension of the early tender deadline of its previously announced exchange offer (the “Exchange Offer”) and consent solicitation (“Consent Solicitation”) to 5:00 p.m., New York City time, on June 2, 2026. A copy of the press release is attached as Exhibit 99.1 to this Current Report and is incorporated herein by reference as if set forth in full. * * * Note Regarding Forward-Looking Statements The Company caution…

Entry into a Material Definitive Agreement. Transaction Support Agreement On May 18, 2026, the Company entered into a transaction support agreement (the “Transaction Support Agreement”) with certain beneficial owners or nominees, investment managers or advisors for beneficial holders of the Existing Convertible Notes who hold approximately 75.2% of the aggregate principal amount of the Existing Convertible Notes (the “Supporting Noteholders”). Pursuant to the Transaction Support Agreement, th…

The filing is about an amendment and restatement of the Company’s equity plan, not a management change.

Results of Operations and Financial Condition. On May 18, 2026, the Company issued a press release reporting its financial results for the quarter ended March 31, 2026. The full text of the press release is attached as Exhibit 99.1 to this Current Report on Form 8-K. In accordance with General Instruction B.2 of Form 8-K, the information contained or incorporated herein, including the press release attached as Exhibit 99.1, shall not be deemed “filed” for purposes of Section 18 of the Securit…