To prepare marinade, combine first 6 ingredients in a large zip-top plastic bag; add chicken. Seal and marinate in refrigerator 3 hours; turn occasionally.
Remove chicken from bag; discard marinade. Thread 4 chicken pieces, 1 piece each red onion and bell pepper, 1 mushroom, and 1 tomato alternately onto each of 8 (10-inch) skewers. Sprinkle with salt and black pepper.
Place kabobs on grill rack coated with cooking spray; grill 12 minutes or until chicken is done, turning once. Serve with Tzatziki Sauce.