It's an issue with drm-free content. Unlock mechanics are a known issue of drm-free content because you cannot control the unlock using an account otherwise it would be drm. But the simple solution is to implement an unlock mechanics by unlock files, which has already been cleverly done for the steam version. What remains to do is to add the unlock files as dlc for those that earned them. Maybe GOG is refusing to add a dlc that cannot be bought. Still, is something Obsidian should have known before deciding to publish on GOG. They at least should provide an official statement about the reason it is still not possible.
There are other easy way, however, like adding an unlock code instead of an unlock file. They only need to update the games and everyone could unlock the contents with the proper code. The code could be a commond code or something like the hash of users email if they really want to control who access the content. So solution exists. Given the game is still supported an new contents are added, there is no reason not to fix that.