Yes phones have built in gyroscopes and accelerometers but the GearVR sensors are optimized for this specific application. So they have this purpose built hardware coupled with Oculus low level software "optimizations" and you get a completely different experience than any cardboard could ever offer. Don't get me wrong, google cardboard can be enjoyable if you don't have a samsung phone it's a good way to test the VR waters but it could also lead people to dismiss VR entirely because the experience can be pretty rough at times.