The Steam version is (almost?) DRM-free as well. You only need it for the first installation and for updates. You don't need the Steam client while playing and you can copy the game files whereever you want them to be, even another computer. Then you can just play the game by clicking on the exe file in the game folder. No Steam needed anymore.
Yeah, GoG Galaxy is literally just Steam with a different name and less games.
Steam is only DRM if the devs choose it to be. Otherwise it's just a downloader that takes the strain off the devs for providing that much bandwidth. In the case of Original Sin ( and Dragon Commander! ), once it's downloaded, you are good to go. You can uninstall Steam at that point, and can still play the game just fine, all you need to do is go to the game folder.
Most indie/devs that support GoG games on Steam are DRM free, including stuff like Paradox games. If it's on GoG/Galaxy, the Steam version will probably be DRM free as well.