Place squash, bell peppers and onion in rectangular baking dish, 13x9x2 inches. Pour dressing over vegetables. Cover and let stand 1 hour to blend flavors.
Heat coals or gas grill for direct heat. Remove vegetables from marinade; reserve marinade. Place vegetables in grill basket (grill “wok”) or directly on grill rack.
Cover and grill vegetables over medium heat 10 to 15 minutes, shaking basket or turning vegetables and brushing with marinade 2 or 3 times, until crisp-tender. Sprinkle with pepper.