The free version of Garry's Mod is available as a download from ModDB.com. Mod DB provides the download free of charge and does not require users to sign up for a membership before downloading.
In addition to the files included in the download from Mod DB, playing Garry's Mod also requires a fully functioning installed copy of "Half-Life 2." Garry's Mod is not a standalone game but a modification to an existing game, and it does not function at all without a working copy of "Half-Life 2."
The free version of Garry's Mod has not received official support from its creators since 2008. The lack of official support for the free version of Garry's Mod makes the modification unstable and prone to bugs, glitches and other problems.