Reposting this from reddit so I can delete my original post (and soon my account hopefully).
GameImage basically just lets you fuss with getting the games working on Desktop first, then turn that into an AppImage file that you can just copy straight to the Deck.
I made a video guide for this a while back, but it's fairly outdated at this point.
That's a great question, and unfortunately I don't remember since I did this like 5 months ago. Sorry :(