Why does the force of gravity do work on a car that rolls down a hill but no work when it rolls along a level part of the road?

on earth... as you get closer to the earths core the gravity becomes stronger.. therefore the bottom of the hill being closer to the core the gravity is stronger then the more distant top of the hill. as for the smooth road the gravity is pulling equally on all sides of the car so it cancels its self out and results in u not moving