Place the yellow squash, zucchini, tomato, olives and onion on a double thickness of heavy-duty foil (about 17 inches x 18 in.). Combine the oil, lemon juice, garlic salt, oregano and pepper; pour over vegetables. Fold foil around mixture and seal tightly.
Grill, covered, over medium heat for 30-35 minutes or until vegetables are tender. Open foil carefully to allow steam to escape.
Transfer vegetables to a serving bowl. Sprinkle with cheese.