GOG is a digital distributor of DRM free games. They used to focus on old games (hence the original Good Old Games name), and do make sure old games work well on new systems, but also have newer and indie games.
The current GOG release is the same as any other digital version of the game. The first download version of DD was released just on GOG; with the
Divinity Anthology release, other digital distributors (like Steam, Gamergate, etc) started carrying it.
