To fix the gta launcher, I followed the steps listed here, except for some step I deemed unnecessary (PROTON_USE_WINED3D=1): https://github.com/ValveSoftware/Proton/issues/37#issuecomment-532684588

Alternatively, you can use the "NOGTAVLAUNCHER" trick; but I had stability issues with it: https://www.reddit.com/r/linux_gaming/comments/d880ds/gta_v_better_than_ever_post_new_launcher/f18fxcn/

To be able to play multiplayer: WINEDLLOVERRIDES=winedbg.exe=d %command%

The graphics look slightly odd compared to in windows. Performance is roughly as good (95%). Multiplayer drops down the framerate from 110fps to 40fps, just like in windows.