GOG doesn't offer the German version, but the UK / international version, so don't install FIXT German Uncut! It's only for the German version (and doesn't even contain German language files, if that's what you're after).

Basically you've got the choice between "fixes only" and "fixes + extra content". As the extra content is developed over years now, restores some parts that were planned to be part of the game anyway and blends seamlessly into the original game, I'd recommend to use the "FULL CUSTOM". You can still customize its installation and deselect certain options. If you're a purist and want to play the game the same way it was released, just with a lot of bugs fixed, install "FIXES ONLY". In the end it's a matter of taste.