That's cleared up, let's address the 'how'. You'll need Garry's Mod 13, the current beta that Garry's been developing for the last year. If you have GMod on Steam you'll now have access to the beta.

Official Garry's Mod spiritual sequel Sandbox is finally available for everyone to try, letting you make any game mode you can think of.

A Garry's Mod player has hacked Kinect to be compatible with the popular HL2 mod.