[zormino]

For lots of modern cars, the users phone is a key, and the car itself is connected, so you could also theoretically share and compare gps location during the unlock attempt.

[drcongo]

I'm no expert, but I'm pretty sure GPS isn't going to have the accuracy needed to prevent a relay attack.

[Merad]

I think the accuracy should be fine, cell phones typically have a GPS accuracy of 5-10 m. The problem is that you won't be able to rely on it. You probably won't have any GPS signal at all in an underground parking garage, and even in an above ground parking deck you will likely have poor or no signal due to being surrounded by steel and concrete.