GOG overlay doesn't work in the game, and I can't even enable it in the configuration because the checkbox is inactive. Therefore i don't see a notification when I get achievements. Anyone else have this issue?

Same here. Presumably because it caused issues shortly after launch it is deactivated. No idea if the developers are still considering it an issue. They seem to be more focused on making the game for Steam players with Steam overlay, considering how one of the patches wanted you to start the Steam client to play the game...