groze: My guess is as good as anyone else's, but I think Galaxy integration takes a bit longer because, unlike Steam, which only needs to be compatible with... well, Steam, Galaxy is supposed to have cross-play, meaning you can play a GOG game with people who bought it on Steam or other distributors.
Like I said, I really don't know, and it's your prerogative, I don't mean to change your mind. I don't even like the Divinity series, so I couldn't care less.
Well, I'd love to be proven wrong. I hate Steam with a passion, but if GOG is essentially going their direction, there's no reason why I should wait to buy it in a month (or longer) when I can get it now on Steam with full multiplayer working.
When someone has tested the GOG coop, let me know if it's truly locked out.