This game works with the latest proton version, and doesn't need the launchoptions. The issue is you need to play around with the controller settings (switch to classic in the menu, install a doff control scheme)

BUT, instead of playing like that I recommend installing Project Reborn following this guide: https://steamcommunity.com/sharedfiles/filedetails/?id=706270825

You need to use the launchoptions for Project Reborn to work. Then I edited 2 config files in the scripts folder:

• GTAVC.WidescreenFix.ini change to match steamdeck aspect ratio "ForceAspectRatio = 16:10" "FrontendAspectRatio = 16:10"
• GInput.ini change the controlset to match GTA IV inputs (my preferred, you can leave as is if you dont mind the 2002 control scheme) "ControlsSet=5"

Im still early game, but no crashes and looks and feels much better than the regular experience.

Stock requires tinkering with controller configs. But install Project Reborn and use the tweaks i suggest and you'll have a better time.