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

This is a basic physics situation. Gravity is a perpendicular force to the earth's surface, or a force that is directed to the earth's center. So when a car is on perfectly level ground it will not move. But when on a hill or any considerable incline it will roll (if the car is not hindered by you pushing on the brake or the car being in park.)

Think of your car as a marble on a level tabletop. Set down the marble and it will sit where you placed it. Now as you raise one end of the table it will roll.

So when your car is in motion it will act simular. When you travel downhill, gravity will push your car to a faster speed (if you don't push the brake.) But if you are on level ground, gravity will not push you to a faster speed. Thus saying it does no work for you, when you want to continue travelling forward.

However, if a car is freely rolling on a level road (with the engine off) it will stop because gravity works with friction to slow the car down. (Now try rolling your marble across your level tabletop slowly, it probably will stop before it falls off. That is friction and gravity hindering it to a stop.)