Vertax: a couple of years ago i bought dungeon keeper 2 from GOG, i dont know how long i have had it, but i stopped playing it a few months ago
anyway i wanted to come back to the game and play it again, the game is not on my list, not on my installed list, not on my owned list
Something is missing here. Not just the game.
If you didn't uninstall the game, it should be on your computer, so you should just launch it and play.
If it's not on your computer, then somebody has uninstalled it, and then it's not an issue with GOG anymore.
Now, if you uninstalled it, then you should probably remember when you did it, and did you try to tag it on GOG shelf or something which might have caused some unwanted consequences.
Also, for an advice for the future: the whole point of GOG is DRM-free and being able to back up everything. Next time, keep installers on your backup drive, so if you somehow happen to lose access to GOG, you can always get the game from your local files.