Garry's Mod

Garry's Mod (Commonly referred to as just Gmod) is a sandbox game released by Facepunch Studios in 2004. It is the game Zuthar13 primarily trolls in.

Overview
While the game supports offline play, many use the online servers to roleplay and play games. That being said, Zuthar primarily goes onto roleplay servers to screw around, and to eventually get banned for breaking the rules.