Spray a large piece of aluminum foil, approximately 8 inches by 15 inches, with cooking spray.
Put the potatoes on the aluminum foil and spray again with cooking spray.
Sprinkle the potatoes with parmesan cheese and garlic salt.
Pull the edges of the aluminum foil to the middle and pinch the ends making a foil packet.
Put the foil packet on the grill over very low heat for approximately 30 minutes or until the potatoes are tender, checking at 15 minutes and then again and 20 minutes.