INS is an independent, on-board system. Its accuracy will primarily depend on the initial position of the aircraft input on the ground before the aircraft moves. Subsequent positions and other parameters are updated using gyroscopes and accelerometers.
GPS is not an on-board system. It has receivers on board the aircraft, however the position is being updated using space vehicles such as satellites. So accuracy will depend on the functioning of the space vehicles. Position is updated using fixes from 3 or more independent space vehicles.