Hello @DumpsterVirus, we're using one issue report per unofficially supported game title and this issue has already been evaluated https://github.com/ValveSoftware/Proton/issues/37#issuecomment-999114189.
Please use that issue report for future feedback on running Grand Theft Auto 5 with Proton.
Compatibility Report
https://gist.github.com/DumpsterVirus/d426ef6e5c0a31a7a392a3fce7c25f95
System Information
I confirm:
[ LOG in GitHub Gist link:
https://gist.github.com/DumpsterVirus/d426ef6e5c0a31a7a392a3fce7c25f95
Symptoms
[Symptom shown in GitHub Gist Link: https://gist.github.com/DumpsterVirus/d426ef6e5c0a31a7a392a3fce7c25f95
" An error has occurred. Please make sure all instances of the Rockstar Games Launcher have been closed and try again. If you continue to see this error, please visit https://support.rockstargames.com"
Reproduction