What is this ?

Garry's mod is basically a sandbox game, very moddable, includes adding new items/models/scripts/gamemodes basically anything you want to add, you can.
The gameplay on the sandbox is a ton of fun, you can spawn items and weld them together, add thrusters and fly on a barrell, add hoverballs and you have yourself a hovering rocket powered barrell, add a seat, and here you go, a mountable rocket powered barrel. add weapons, lights basically anything.