There is no difference between both versions. Local publishing is available in both and the Steam publish is simply disabled when no connection is available. (You could technically run the GOG version while logged into Steam and still upload to Steam)
As has been mentioned on a couple of posts, we have no exact time ourselves on when the GOG version is made available. I've seen them actively commenting on this request on their forums (
https://www.gog.com/forum/divinity_original_sin_2#1505810906) and I'm sure they'll be able to give you a better time frame on it.
