mistermumbles: So how do you exploit that for those to show up? ... I do hope that Homeworld will indeed be coming here. :)
As long as you have the game's ID number, you can go about adding it to GOG Galaxy this way:
https://www.gog.com/forum/general/adding_external_games_to_galaxy You can't do much since it's technically an unauthorized copy. You don't need the real files, all you need is the .info file and any .exe file to link to it, and it'll be added to your Galaxy list.
The trick is getting the number in the first place, of course. If anyone wants to try for themselves:
"Seawolf" - 1437731013
Homeworld Remastered Collection - 1448551424
Battlestation: Harbinger - 1460547741