There should be an error code (E1, E2, E3, etc.) for one of these cases, check here and see which one can help and try doing what the article recommends:
https://support.gog.com/hc/en-us/sections/360003883478-GOG-GALAXY-Error-codes?product=gog

I tried everything to fix this to no avail.
The game IS installed just fine, its Galaxy that fails itself.
I had to copy the game files to another place then remove the game from Galaxy completely to stop it from constantly downloading and failing OVER AND OVER AND OVER!

Hello everyone,

did anyone find a workaround to get the game in Gog? I get an E2 error and tried the proposed solutions in the Gog documentation to no avail too, it's a little frustrating.
I guess the installation folder may be used to play outside of Gog but I'd like it to work.

Edit: nearly 3 months later and after reading carefully the documentation, I managed to install the game and add it in Gog following the second part of this doc:
support.gog.com/hc/en-us/articles/360013537357-GOG-GALAXY-Installation-failed-E2-Update-failed-E2-Failed-to-fi nalize-the-update?product=gog

You go on www.gog.com, log in with your account then download "Offline backup game installers" and then run it.
It's the Gog installer so it will install the game in a directory known by Gog Galaxy and will automatically see the game has been installed and add it to your list. It's as if it was installed using Galaxy, yeah!

Upping post with my edit above, just in case