The "Gas-Operated Semi-Automatic Firearms Exclusion Act," or GOSAFE Act, aims to amend title 18 of the United States Code by restricting the possession and transfer of specific firearms and ammunition feeding devices. It defines a semi-automatic firearm as one that fires a cartridge, uses energy to cycle the action, and requires a separate trigger pull for each shot, excluding machineguns. A firearm is considered gas-operated if it harnesses gas from a fired cartridge to cycle its action through various mechanisms like pistons, direct impingement, blowback, or recoil. The bill makes it unlawful to import, sell, manufacture, transfer, receive, or possess certain firearms and devices. This includes firearms on a prohibited list, modified firearms that operate as prohibited ones, parts designed to create prohibited firearms, and devices that materially increase a semi-automatic firearm's rate of fire or approximate a machinegun's action. Additionally, it prohibits the import, sale, manufacture, transfer, or receipt of large capacity ammunition feeding devices (those holding more than 10 rounds, or 15 for handguns, and not .22 rimfire) and the possession of such devices manufactured after the bill's enactment. Exceptions to these prohibitions include transfers to or possession by the United States government, state or tribal entities, and licensees under the Atomic Energy Act for security purposes. Lawfully manufactured and transferred gas-operated semi-automatic firearms possessed before the enactment date are also exempt, with a limited transfer exception to immediate family members after a licensed dealer facilitates the transfer. All exempted firearms and devices must be marked as prescribed by the Attorney General. The Act mandates the Attorney General, through the Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF), to publish and update a list of prohibited gas-operated semi-automatic firearms. Manufacturers of new semi-automatic firearm designs must apply for approval, providing detailed specifications and a physical sample, with a review process and potential appeals. The bill establishes a Firearm Safety Trust Fund , funded by existing taxes and new application fees, to support the implementation of these provisions and the National Firearms Act. Violations carry penalties of fines, imprisonment, or both, with enhanced penalties if a prohibited firearm is used in another federal offense. The bill also allows Byrne grants to be used for buy-back programs for these restricted firearms and devices.