Garry's Mod (commonly abbreviated as GMod) is a sandbox physics game created by Garry Newman  and developed by his company, Facepunch Studios.  Garry's Mod was originally a mod for Valve Corporation's Half-Life 2, but was later made into a standalone release in 2006 for Microsoft Windows published by Valve Corporation. Later updates saw an OS X, added in 2010, and a version for Linux in 2013. It is often used for machinima, SuperMarioGlitchy4's bloopers included. SMG4 often uses the game engine to shake models around spastically for comedic effect. Originally Garry's Mod was used for characters such as Teletubbies or other things that aren't in Super Mario 64. Now SMG4 uses it for most of the characters. In more recent videos, they are mostly seen in their G. Mod forms.
A common criticism of SMG4 is the overuse of Garry's Mod in some videos.