Prepare a medium fire. Alternate squash and onion slices in rows on a large sheet of heavy-duty aluminum foil. Sprinkle vegetables with garlic salt, salt, and pepper. Dot with butter. Fold foil securely to seal.
Place foil package on grill set 4 to 6 inches from coals. Cook without turning until vegetables are crisp-tender when pierced with a fork, about 15 to 20 minutes