Heat coals or gas grill for direct heat. Cut 18x18-inch piece of heavy-duty aluminum foil. Spray with cooking spray. Place cauliflowerets, carrots and onion on center of foil. Drizzle with oil; sprinkle with garlic pepper and salt. Stir gently to mix.
Fold foil over vegetables so edges meet. Seal edges, making tight 1/2-inch fold; fold again. Allow space on sides for circulation and expansion.
Cover and grill packet 4 to 6 inches from medium heat 13 to 18 minutes or until vegetables are tender. Place packet on serving platter. Cut large X across top of packet; unfold foil. Gently stir in olives and basil.