I have no idea about Tesla specifically but normally you'd also you cell and wifi information (in their case probably also information from other teslas around)

I have no idea about Tesla specifically but normally you'd also you cell and wifi information (in their case probably also information from other teslas around) and additionally you have accelerometer and the whole "self-driving" computer to estimate where the car is and where it's going. It's also a known attack vector and likely covered because GPS signal is really weak so it's easy for somebody outside the car to try to make a mess.