When a unit boards a vehicle, their position (obtained with the getpos command) is actually back where they got in!!
So as far as setting the camera to target the gunner, it can't really be done (as far as I know
).
It is however possible to target co-ordinates, rather than a unit/vehicle. So what you could do is work out where the gunner is in relation to the centre of your vehicle, and target those co-ordinates