In what year was Garry's Mod released as a standalone title on Valve's Steam distribution service?

Garry's Mod was released as a standalone title on Valve's Steam distribution service in 2006 because it was the year when the game reached its full potential and gathered a substantial player base. The sandbox physics game had been in development as a modification for Valve's Half-Life 2 since 2004, but it was not until 2006 that it was officially launched as a standalone game on Steam, providing players with an extensive toolset to create and manipulate their virtual worlds using the Source engine.