Garry's Mod, or GMod for short, and stylized as garry's mod or gmod, is a "sandbox" mod for the first-person shooter computer game Half-Life 2. It was mainly created by Garry Newman.
Garry's Mod started out as a source mod for Half-Life 2. It allowed the player to use any of the characters (strangely enough, not Gordon Freeman) as Ragdolls or NPCs, and as time went on, Counter Strike: Source, Team Fortress 2, and other Valve produced games. Playermodels are the models your character uses, but aren't spawnable.
As time went on, GMod became its own individual game, and would later be sold on Steam. Thus, the Workshop was born. A replacement for the Toolbox, the Workshop allows people to submit models, props, maps and more and have others "subscribe" to them, which is an alternate saying to "downloading" them. So far, there are thousands of Ragdolls, NPCs, Playermodels, Dupes, Maps, Saves, and yes, videos, on the workshop. Many of these include models from popular series', such as Super Mario, Halo, The Walking Dead, Five Nights at Freddy's (which has become unbelievably popular in such a small amount of time), even hexed models/props/effects from other valve games, such as Team Fortress 2, Half-Life 2 and Portal 2. Other noticeable things are weapons, such as the M9K Weapons, GBombs and Trouble in Terrorist Town weapons, and Vehicles. SCars had originally set Gmod's future of driving vehicles, featuring full customization on the vehicles themselves, or the TDMCars, which would also lead the way for ported models from various games.
Another neat feature was that you can add scripts, such as VCmod, a lighting script for various vehicles across the workshop. These scripts were mostly on Coderhire, until they closed up shop and script sellers had to move to another site. That site was scriptfodder You can currently find these Gmod scripts there.
The current version of Garry's Mod is version 14 (Gmod 14), which still receives regular updates via Steam. An older, unsupported version, 9.0.4, is available as a free download at download.com among other places. It requires at least one Source engine game in the user's account to play, and therefore is offered in a reduced-price bundle with Counter-Strike: Source or Team Fortress 2 for those who do not meet that criterion.