So far, I've only found instructions on how to get this mod running in Steam. But I can't find out how to run this mod in Gog Galaxy so that the mod runs when I click the "Play" button. Has someone figured out how to do this?

Nevermind, I just figured out how! Here's how to do so if anyone is wondering how to run Et Tu in Gog Galaxy. It's so simple!

Click the rightmost button with the two circles on lines and select 'Manage Installation'. Click on 'Configure'. Click 'Features' in the menu that opens up. Check the box 'Custom executables / arguments". Click on 'Add another Executable / arguements'. Locate the file in your Fallout1in2 folder that runs the program and choose it. Now click on the 'Default executable' button. Now when you click 'Play' on the main page of the Fallout game, it will run the mod!

This also works for the FIXT mod!