Could someone else have tried the game on that system a couple of years ago? Check the profile name, and characters names (if they were changed from the default); if they correspond to names commonly used by someone who had access to your computer, that's where they came from.
Check the creation dates for the folders in the path '..\Documents\Larian Studios\Divinity Original Sin Enhanced Edition\PlayerProfiles\<ProfileName>\Savegames_patch'.

Try deleting the My Documents D:OS EE folder in Windows Explorer, restart the game, and see if Galaxy downloads anything.
If so, Galaxy may have got its wires crossed somewhere. Contact GOG support; technically you could delete everything in-game and possibly disable cloud save support (if you do not need it), but that wouldn't help with tracking down what happened.