Timmareus: I managed to fix it by going into the folder GOG downloads updates into and simply copypasting the downloaded files into the game folder, overwriting the old ones. Try this at your own risk and don't blame me if this ends up messing up your install, but it worked for me. Maybe back up your save files first just in case. It seems the failure is just in the final step of actually putting the files where they need to be.

I shut down Galaxy first so it doesn't get confused. No idea if that's necessary.

If you don't know where GOG puts your update files, you can check it under Settings - Downloads & Updates or something along those lines in the GOG client.

chew-ie: Good idea! I tried it - but triggered an even bigger download (27.80 GB).

My steps in details:
- closed Galaxy (tray icon -> close)
- archived the whole game directory to my backup drive
- copied everything from !temp/ to the main game directory
- started Galaxy
- used verify / repair
- start the game

Maybe Owlcat did a fix in the meantime ¯\_(ツ)_/¯

Will see if that works at the end once the download finishes (somewhen tonight...).

Keep posting your experiences! :)

Edit: file validation failed after checking 3GB - giving it another go, then try a complete reinstall - or patch from the offline installers (whatever goes faster)

Timmareus: Yeah, my method only works as long as you keep Galaxy off - if you turn it back on it redoes the whole update, as I have learned to my cost. If you're fine with not updating/installing any other GOG games, you can just keep Galaxy off for a couple days while someone figures out a proper solution.

Thankfully GOG is not like steam or other services where you can't start the game without involving the launcher in some way. Not an ideal solution, but if you just wanna play the game it should work.