When people note that some aircraft (even light aircraft) already have GPS, remember that these are not bought at Radio Shack and pasted onto the panel. They are integrated into the aircraft systems, and so proper precautions are taken at the design stage, so they do not affect each other.
Now I don't know, but MAYBE Garmin has engineered their handheld aviation units so that they don't affect plane systems. That's the only way I can imagine you could safely use it in a Cessna. In this case, the equipment would have met the relevant certification standards. This is unlike waking into a store and picking a unit off the shelf and using it in a plane.