From GMod Wiki
The Vehicles Menu is part of the Game Menu and it is where you can spawn drivable vehicles for your game.
Garry's Mod comes with a wide selection of vehicles as default:
- Airboat Seat
- Jeep Seat
- Office Chair (1)
- Office Chair (2)
- Wooden Chair
There are also a large number of custom vehicles available form other sources such as http://garrysmod.org.
The objects is welded to the vehicle and the Paint Tool sets the vehicle's opaque factor to zero. This, combined with the No Collide, effectively makes the vehicle invisible. This results in an illusion that makes the player appear to "drive" the object.