Not the GP, but the areas I use windows still for gaming is VR since I apparently have to do a lot of hacks to get it to work on my system that I'm not willing to try and fail. And then I play Lethal Company and similar, but that starts to struggle and have problem when you want to do anything modded since the modding tool available have trouble with it not being windows and break often when run though proton/wine. I've eventually got it to work, but only with hacky work arounds like changing the launch options in steam since the "Start Modded" button refused to work. I think most things running in electron fail to run through proton/wine right now (on my machine at least.)
In the end if I was playing a game alone I'd hack on it for days just to get it working so I can enjoy it on linux (hacking away at the game specifically not the linux install like needed for VR). But when I want to play a game with friends spending days (or even hours) isn't an option since they'll be onto something else by then.
I'm using r2modman on Arch for Lethal Company and RoR2, has that worked for you? I have it and steam installed from the AUR/Repo's, I suspect flatpak would cause issues.
I definitely get just keeping windows around. I'm lucky enough that things usually have Just Worked, and I don't use VR.
r2modman is also what I use, at this point I think I have 3 versions, Windows Exe, AppImage and pacman. I don't remember what one worked in the end but I remember one couldn't find the game even if manually located. One failed to render anything other than a black image and the last (working) one is just really slow until I get into the actual mod selection, then as before the working one can't actually start the game for some reason, it does give you the parameters you need to throw into steam to start the modded version instead. Maybe I'll poke at it again soon, I just don't want to run into issues when my friends are already ready to play the game.
In the end if I was playing a game alone I'd hack on it for days just to get it working so I can enjoy it on linux (hacking away at the game specifically not the linux install like needed for VR). But when I want to play a game with friends spending days (or even hours) isn't an option since they'll be onto something else by then.