Is the Gauss Cannon just bouncing around or is it being aimed?
It's being aimed by Cortana, who has a remote module wired to Master Chief's vehicles from the Forward Unto Dawn. Good question! 
So is she aiming anywhere she wants or is Chief giving suggestions?
Cortana would basically act like the Gunner AI does now.
I figured as much because if you could aim in the Warthog it would be hard to drive.