I tried a bunch of different stuff but couldn't past the splash page. Turns our that the issue was never the proton version or the startup command for me.

I had to create an Options.ini file. (Options not option). You need to navigate to: /.steam/steam/steamapps/compatdata/2229870/pfx/drive_c/users/steamuser/Documents/Command and Conquer Generals Data And create a file called Options.ini Here's what i put in the file: AntiAliasing = DrawScrollAnchor = FirewallBehavior = 0 GameSpyIPAddress = 0.0.0.0 Gamma = 50 IPAddress = 0.0.0.0 IdealStaticGameLOD = High LanguageFilter = false MoveScrollAnchor = MusicVolume = 55 Resolution = SFX3DVolume = 79 SFXVolume = 71 ScrollFactor = 50 SendDelay = no StaticGameLOD = High UseAlternateMouse = no VoiceVolume = 70

Without this file I couldn't get the game to work. Also if you're using the version of steam from snap I would reccomend removing it and using sudo apt install steam.

I could not get this game to launch without tinkering and adding the Options.ini