Garry's Mod это Jastin Biber Forever!!!
You can spawn objects and connect them with any of the various constraints 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.
If you're not too great at construction - don't worry! You can place a variety of characters in silly positions, or have a quick battle with other players.
It's also highly modifiable, which allows third party users to make their own additions. Many new and exciting additions are added to Garry's Mod every day!