Garry’s Mod can still regularly be found in Steam’s list of the top ten most-played games at any given time, but before it stood alone, the game began life as a Half-Life 2 mod. Garry’s Mod transformed that sleek, straigtforward singleplayer shooter into… well, the full game’s description sums it up very well.
“Garry’s Mod is a physics sandbox. There aren’t any predefined aims or goals. We give you the tools and leave you to play. You spawn objects and weld them together to create your own contraptions—whether that’s a car, a rocket, a catapult or something that doesn’t have a name yet—that’s up to you. You can do it offline, or join the thousands of players who play online each day.”
Beyond those thousands of daily players, you’ll find hundreds of thousands of Garry’s Mod models, modes, and assets in Steam Workshop. Beyond games, people have used it to create all sorts of wild webcomics, videos, and—uh—McDonalds. The only limit is your imagination.