Half Life 2 (Often abbreviated by fans and media as HL2) is a 2004 First Person Shooter (FPS) Created by Valve. In addition, it's the game which Garry's Mod is derived from, the latter originally being a mod for the former, before being made into a game in its own right. Half Life 2 is set in a dystopic near-future, putting the player, Gordon Freeman, against an alien force called the Combine.
Due to Half Life 2 being the base game for Garry's Mod, no matter what source game the user owns, even if they don't own Half Life 2, they will have the textures, models, NPCs and other materials such as props and vehicles. However, HL2 must be owned by the user if they are to access the game's many maps.
The game also has two sequels, known together as Half Life 2: Episodes 1 & 2. The content of the two episodes won't be present unless the user owns them.
GMod Characters adapted from Half Life 2 Edit
Perhaps the most well known character in the GMod fandom originating from HL2 is Dr. Hax, who features in DasBoSchitt's GMod Idiot Box series, and is based off HL2's primary Antagonist, Dr. Breen.