Spray grill rack with nonstick spray. Preheat grill to medium high or prepare medium-hot fire.
Stir together lemon zest and juice, oil, garlic, salt and pepper in a large bowl. Add chicken, zucchini, yellow squash, tomatoes, and onion. Toss to coat.
Thread onto approximately 16 (6-inch) skewers. Please note: If using wooden skewers, soak in water 30 minutes before threading on the chicken and vegetables.
Place skewers on grill rack and grill, turning frequently, until browns and cooked through. Approximately 10-12 minutes.
Transfer to serving plate and sprinkle with basil. Serve with lemon wedges.