GOG offline installation files are even less organized than I thought. If you install the game from there, after long hours of painstakingly downloading the small chunks, you'll get a big Data Mismatch displayed all over the launcher.

If you want to get rid of that, delete the two files GOG thought useful to add:

- EULA.txt
- webcache.zip

Or move them to a backup location.

I joined the image to this post, but those forums seem buggy as well, so here it is:

https://ibb.co/G4BKqnmP