Gossamer Bio, Inc.

Item 1.01. 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 Collaboration and License Agreement, dated May 3, 2024, entered into by Chiesi and Gossamer (the “License Agreement”), subject to survival of certain provisions, and provide for assistance and cooperation in connection with certain wind-down activities conducted by or on behalf of Chiesi; (b) to provide for the reacquisition by Gossamer of seralutinib assets (including by termination of licenses granted under the License Agreement by Gossamer to Chiesi and assignment or transfer or license of related assets, including regulatory filings and certain intellectual property rights related to seralutinib, by Chiesi to Gossamer) and worldwide development and commercial rights to seralutinib, including control of pulmonary arterial hypertension (PAH), pulmonary hypertension associated with interstitial lung disease (PH-ILD) and potential future indications, and (c) to provide for certain post-termination payments and related obligations in consideration of the rights granted under the Rights Reacquisition Agreement. Under the Rights Reacquisition Agreement, (a) Chiesi will pay to Gossamer $5 million (the “Chiesi Amount”) within 10 days after the date of the Rights Reacquisition Agreement as reimbursement of outstanding development costs yet to be reimbursed or incurred, and (b) in consideration of the development activities conducted by or on behalf of Chiesi and costs and expenses incurred by Chiesi under the License Agreement, as well as the return of related seralutinib assets, Gossamer has agreed to (i) make certain success-based milestone payments to Chiesi and (ii) pay royalties on net sales of certain products previously licensed under the License Agreement up to a capped amount, after which no further payment obligations would be due under the Rights Reacquisition Agreement. In connection with the reacquisition of seralutinib rights under the Rights Reacquisition Agreement, Chiesi has assigned and licensed certain intellectual property rights owned or jointly owned by Gossamer and Chiesi that cover the products originally licensed under the License Agreement. Such licenses may be revoked by Chiesi in the event of a breach by Gossamer of its undisputed payment obligations under the Rights Reacquisition Agreement, subject to certain specified cure periods. In consideration for such assignment and license, in certain specified circumstances, Gossamer is obligated to use commercially reasonable efforts to continue to develop and/or commercialize certain products previously covered by the License Agreement. The parties to the Rights Reacquisition Agreement have also agreed to the survival of indemnity obligations for claims arising under the License Agreement, as well as a mutual release of all claims under the License Agreement other than those that may arise under the surviving indemnity obligations or claims raised under the Rights Reacquisition Agreement. The foregoing description of the Rights Reacquisition Agreement is not complete and is qualified in its entirety by reference to the full text of the Rights Reacquisition Agreement, a copy of which is filed as an exhibit to this Current Report on Form 8-K.

Item 1.02. Termination of a Material Definitive Agreement. The disclosure set forth in Item 1.01 of this Current Report on Form 8-K with respect to the termination of the License Agreement is incorporated into this Item 1.02 by reference.

Item 2.02. 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 Item 2.02 shall not be deemed “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, as amended, nor shall it be deemed incorporated by reference in any filing under the Securities Act of 1933, as amended, except as shall be expressly set forth by specific reference in such a filing.

Item 7.01 Regulation FD Disclosure. On July 27, 2026, the Company issued a press release announcing its entry into the Rights Reacquisition Agreement and the related termination of the License Agreement and providing certain FDA regulatory and business updates. 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, shall not be deemed “filed” for purposes of Section 18 of the Exchange Act, or otherwise subject to the liabilities of that section, nor shall it be deemed to be incorporated by reference into any filing under the Securities Act or the Exchange Act, whether made before or after the date hereof, except as expressly set forth by specific reference in such filing to this Current Report on Form 8-K.

Item 8.01. 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 effect observed in PROSERA as review issues rather than filing issues. On that basis, the Company intends to submit an NDA supported by one adequate and well-controlled study (Phase 3 PROSERA) plus confirmatory evidence (Phase 2 TORREY and supportive analyses). If the NDA is accepted for filing, seralutinib could be eligible for an FDA approval decision in the third quarter of 2027. While the meeting minutes reflect FDA feedback as of the meeting date, the FDA's ultimate determination on approvability will be made upon review of the complete NDA.