"Garry's Mod is a Source-based sandbox game. Originally, Garry's Mod was a mod for the game "Half Life 2" by Valve. The mod was so successful with the Half Life 2 community that it was released as its own game in 2004. The secret of Garry's Mod's success is that it is based on the player's ability to program addons and publish them on Steam. This way, players are able to personalise their game with unofficial content of their choice and perfect the expirience. To date, Garry's Mod is available for download on Steam.
There is no such thing as a goal in Garry's Mod. You can do whatever you want.