None of the guides here worked for me, so I went looking for something to mess up in the game folder. I'm a programmer, but this requires zero skills.

• Open a Lua file for editing. Let's pick toolmenu.lua in the directory
  \SteamLibrary
  \SteamApps
  \common
  \GarrysMod
  \garrysmod
  \gamemodes
  \sandbox
  \gamemode
  \spawnmenu
  
• Make a backup.
  
• Add this anywhere:
  include( 'foo.lua' )
• Make 500 copies of that line. (In Notepad++, press and hold CTRL + D.)
  
• Start the game, load a sandbox map and watch the magic happen.

Don't forget to restore the backup.