As of June 2015, there is no way to play "Garry's Mod" for free. It is available for $4.99 on Steam, a digital distribution service, with versions available for Windows and Mac OSX.
"Garry's Mod" was created in 2004, and was free to play until Nov. 27, 2005, when the last new, free version was released. In 2006, the game was added to Steam, where it has since sold over six million copies. "Garry's Mod" was named after its creator, Garry Newman, who originally designed the game as a mod for "Half-Life 2". "Garry's Mod" eventually progressed to being a stand-alone game, with no official ties to "Half-Life 2."