Garry's Mod is a sandbox video game, originally created as a Mod for Half-Life 2. SMG4 is well known to use it.
Use in MarioMario54321's videos
Garry's Mod was only seen in MM54321's video Moon of Ender II: Endermite Swarm. The footage originated from another source, featuring Dr. Eggman waving a med PAC shouting "Pingas".
MM claims he lacks the game, but hopes to use it for future videos.