Subsequent Events	6 Months Ended
Jun. 30, 2026
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events Rights Reacquisition Agreement On July 23, 2026, the Company, Gossamer Bio USA, Inc. (formerly GB002, Inc.) and Gossamer Bio 002 Ltd., on the one hand, and Chiesi, on the other hand, entered into a Rights Reacquisition Agreement (the “Rights Reacquisition Agreement”), under which the Company and Chiesi have agreed (a) to terminate that certain Chiesi Collaboration 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 the Company of seralutinib assets (including by termination of licenses granted under the Chiesi Collaboration Agreement by the Company 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 the Company) 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 paid $5 million (the “Chiesi Amount”) to the Company as reimbursement of outstanding development costs, and (b) in consideration of the development activities conducted by or on behalf of Chiesi and costs and expenses incurred by Chiesi under the Chiesi Collaboration Agreement, as well as the return of related seralutinib assets, the Company 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 Chiesi Collaboration 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 the Company 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 the Company 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, the Company is obligated to use commercially reasonable efforts to continue to develop and/or commercialize certain products previously covered by the Chiesi Collaboration Agreement. The parties to the Rights Reacquisition Agreement have also agreed to the survival of indemnity obligations for claims arising under the Chiesi Collaboration Agreement, as well as a mutual release of all claims under the Chiesi Collaboration Agreement other than those that may arise under the surviving indemnity obligations or claims raised under the Rights Reacquisition Agreement. Increase in Authorized Shares and Proposed Reverse Stock Split On July 14, 2026, at the Company’s special meeting of stockholders (the “Special Meeting”), the Company’s stockholders approved, among other proposals, (i) an amendment to the Company’s Amended and Restated Certificate of Incorporation (the “Charter”) to increase the number of authorized shares of common stock from 700,000,000 to 4,000,000,000 and (ii) a series of alternate amendments to the Charter to effect a reverse stock split of the issued and outstanding shares of common stock and a proportionate reduction in the number of authorized shares of common stock. Accordingly, on July 14, 2026, the Company filed a certificate of amendment (the “Charter Amendment”) to the Charter with the Secretary of State of the State of Delaware to increase the number of authorized shares of its common stock from 700,000,000 to 4,000,000,000 in order to support, among other things, the additional share issuances of common stock issuable upon conversion of the Company’s newly issued 2030 Notes and exercise of the Company’s newly issued Purchase Warrants, each issued as part of the Exchange Offer, and under the Restated Plan. The Charter Amendment became effective upon filing.

On July 23, 2026, the Company, Gossamer Bio USA, Inc. (formerly GB002, Inc.) and Gossamer Bio 002 Ltd., on the one hand, and Chiesi, on the other hand, entered into a Rights Reacquisition Agreement (the “Rights Reacquisition Agreement”), under which the Company and Chiesi have agreed (a) to terminate that certain Chiesi Collaboration 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 the Company of seralutinib assets (including by termination of licenses granted under the Chiesi Collaboration Agreement by the Company 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 the Company) 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 paid $5 million (the “Chiesi Amount”) to the Company as reimbursement of outstanding development costs, and (b) in consideration of the development activities conducted by or on behalf of Chiesi and costs and expenses incurred by Chiesi under the Chiesi Collaboration Agreement, as well as the return of related seralutinib assets, the Company 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 Chiesi Collaboration 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 the Company 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 the Company 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, the Company is obligated to use commercially reasonable efforts to continue to develop and/or commercialize certain products previously covered by the Chiesi Collaboration Agreement.

The parties to the Rights Reacquisition Agreement have also agreed to the survival of indemnity obligations for claims arising under the Chiesi Collaboration Agreement, as well as a mutual release of all claims under the Chiesi Collaboration Agreement other than those that may arise under the surviving indemnity obligations or claims raised under the Rights Reacquisition Agreement.

On July 14, 2026, at the Company’s special meeting of stockholders (the “Special Meeting”), the Company’s stockholders approved, among other proposals, (i) an amendment to the Company’s Amended and Restated Certificate of Incorporation (the “Charter”) to increase the number of authorized shares of common stock from 700,000,000 to 4,000,000,000 and (ii) a series of alternate amendments to the Charter to effect a reverse stock split of the issued and outstanding shares of common stock and a proportionate reduction in the number of authorized shares of common stock. Accordingly, on July 14, 2026, the Company filed a certificate of amendment (the “Charter Amendment”) to the Charter with the Secretary of State of the State of Delaware to increase the number of authorized shares of its common stock from 700,000,000 to 4,000,000,000 in order to support, among other things, the additional share issuances of common stock issuable upon conversion of the