So i changed the pads today but when i took the caliper apart the rotor quite literally fell off so i thought that it would be supported by the caliper so i changed pads etc etc and go to reassemble but not there rotor can move around is there any physical mount to the hub for the gt4 rotor

Cheers

Thats part of why you are supposed to torque the lug-nuts. When you tighten down the lug-nuts it sandwiches the rotor between the wheel and the hub, with the wheel studs supporting it.
It is normal on many vehicles the have some play in the rotor with the wheel removed.

ok well there is about 1-2mm on each side of the rotor to the pad, once i use the brakes the piston should extend and fix that gap correct?

Yes.... pump the brakes before you drive it'll tighten up.