So an air traffic controller is basically a GPS navigator for the pilots?

Not at all... As the name suggest, the role of ATC is to control air traffic so airplanes don't crash into each other. It has nothing to do with telling pilots "how to find their way", just ensuring they maneuver at certain altitudes, speed and direction so they are kept at a safe distance from each other, particularly near airports with lots of planes landing and taking off. ATC also deals with on-route traffic but in that regard is more to keep track of who is where, again ensuring a safe distance is kept among airplanes in various routes...