Larian Studios Forums Divinity - Original Sin Divinity - Original Sin - Technical Issues Connection failed server has wrong version

Yes, the version number is the issue.
A handful of people ran into that with D:OS; most fixed it by reinstalling the game to a different location/drive; a couple with both the Steam and GOG versions (Kickstarter backers) simply switched to the opposite version.

Try a clean reinstall of the GOG version, to a different location (a different drive, if available).