Playing it now, It seems that x64 Chromium works properly on Proton now, which is awesome, No more compromised GMod! To celebrate this awesome news, I'll give y'all a quick n' dirty Tutorial on how to get it going. It's similar to on Windows, but there are a few additional steps. I'm assuming you have installed this before under Windows and know how to use a computer.

• First, Make sure you're using the x86-64 Chromium branch. Recent updates seems to have fixed broken menus.
  
• Next, Grab the G64 installer for Windows from the website.
  
• Then you'll need to install "Protontricks" either using the package manager or flatpak (in the case of SteamOS)
  
• Run Protontricks, Select Garry's Mod's prefix, then hit Next as "select default prefix" is selected.
  
• Run explorer, Next

A window will spawn in the top left corner. Navigate to /home/deck/downloads/ and then the .exe you downloaded. double click to run it, install it.

NOTE: "deck" is where your username would be, unless you're using SteamOS.

• Now the ROM File. rename the ROM to simply "sm64.z64", and then drop it into the addons folder.
  
• place it under
  "/home/deck/.local/share/Steam/steamapps/common/GarrysMod/garrysmod/addons/"
  
• NOW, Under g64 settings, either you can browse to the file using Wine's file picker, or simply just paste "/home/deck/.local/share/Steam/steamapps/common/GarrysMod/garrysmod/addons/sm64.z64"

Now you too, can toy around with Mario, just like any Windows Elitist would!

Enjoy! =D