Posted August 09, 2022
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1. Go to your Fallout 3 page in your GOG Galaxy 2.0 library (not sure how it is in GOG Galaxy 1.0, so just wanted to be clear).
2. Click on the Configure icon—the one with two dots, each connected to a horizontal line.
3. Go to Manage Installation > Configure, and checkmark "Custom executables / arguments" and make a new argument.
4. Under File 4, select C:/.../Fallout3.exe.
5. Click on "Default executable".
Now GOG Galaxy should open up Fallout3.exe instead of FalloutLauncher.exe when you click "Play".
Not sure if this will help anyone; all I know is that it made it work for me, in addition to the solutions stated before this comment.
Good luck, everyone!
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