That's a good idea, however, `pacman -Qss gog-` should be sufficient to display what GOG packages you have (obviously it won't display your other games, but you should be able to pipe it through `pacman -Qg`, too).
I'm not aware of any other game that's actually in the 'games' group. In fact pacman told me that the 'games' group wasn't found, and I have a fair number of games installed, even ones from the official repositories.
So I think our packaging standards already make these games as easy to search, list and catalogue as they need to be. I don't think we really need
to add them to the games group.
Of course, nothing prevents you from adding them to the group prior to installation if that works better for you.