Create the link:

Either:

• Use GModCEFCodecFix[github.com], it fixes codecs and the crashing issue
  Note: If you replaced Chromium libraries in the past, restore the ones provided by the game
  
• Link global Chromium libraries (any flavour)
  
  1. Back up existing libraries
     
     mv /path/to/GarrysMod/bin/linux64/libGLESv2.so{,.bak} mv /path/to/GarrysMod/bin/linux64/libEGL.so{,.bak}
  2. Link global libraries to game library path
     
     ln -s /usr/lib/chromium/{libGLESv2,libEGL}.so /path/to/GarrysMod/bin/linux64/

Note: This is optional if GModCEFCodecFix[github.com] is used

Install robotboy655's gmod-lua-menu[github.com]

1. Back up the original menu
   
   mv /path/to/GarrysMod/garrysmod/lua/menu/menu.lua{,.bak}
2. Clone gmod-lua-menu someplace else
   
   cd /tmp/ git clone https://github.com/robotboy655/gmod-lua-menu
3. Copy materials and lua directories over to the game installation
   
   cp -r gmod-lua-menu/{materials,lua} /path/to/GarrysMod/garrysmod/

Add OpenSSL 1.0.1 to the game's library path

1. Obtain OpenSSL 1.0.1u[www.openssl.org]
   
2. Extract the archive and compile it
   
   tar xzf openssl-1.0.1u.tar.gz cd openssl-1.0.1u ./config shared make -j$(nproc)
3. Copy the compiled libraries to your game installation
   
   cp {libssl,libcrypto}.so* /path/to/GarrysMod/bin/linux64

This section is not complete yet, and it will likely be its own guide.

Until either is complete, refer to the equivalent guide for Counter-Strike: Source, but with these differences:

• Substitute the file paths of both .res files with GarrysMod/garrysmod/, and replace both of them. Note file capitalisation differences
  
• The Missing Glyphs section is redundant until "SetLocale('en_US.UTF-8') failed" is fixed

https://steamcommunity.com/sharedfiles/filedetails/?id=2829864889

Steam Overlay was disabled by the developers due to frequent crashes. Until the situation improves, there wouldn't be a reason to hack it in as it would result in an even worse experience.

There is no known fix for this!

For some reason, this error happens, evident by console output:

SetLocale('en_US.UTF-8') failed. Using 'LC_CTYPE=en_US.UTF-8;LC_NUMERIC=C;LC_TIME=en_US.UTF-8;LC_COLLATE=en_US.UTF-8;LC_MONETARY=en_US.UTF-8;LC_MESSAGES=en_US.UTF-8;LC_PAPER=en_US.UTF-8;LC_NAME=en_US.UTF-8;LC_ADDRESS=en_US.UTF-8;LC_TELEPHONE=en_US.UTF-8;LC_MEASUREMENT=en_US.UTF-8;LC_IDENTIFICATION=en_US.UTF-8'.
You may have limited glyph support.
Please install 'en_US.UTF-8' locale.

• AppFramework : Unable to load module engine_client.so | Issue #5056[github.com]
  
• Game crash on Steam Linux Runtime | Issue #5092[github.com]
  
• Startup Crash on Linux | Issue #5096[github.com]