Heat grill. Wrap pita flat breads in foil; set aside. In small bowl, combine mayonnaise and garlic; blend well. Refrigerate until serving time. Sprinkle Greek seasoning over chicken breast halves.
When ready to grill, place chicken on gas grill over medium heat or on charcoal grill 4 to 6 inches from medium coals. Cook 12 to 15 minutes or until chicken is fork-tender and juices run clear, turning once.
During last 5 minutes of cooking time, place flat bread packet on grill; heat until flat breads are warm, turning foil packet once.
To serve, spread warm flat breads with mayonnaise mixture. Thinly slice chicken. Layer each flat bread with chicken, cucumber and tomato; roll up. If desired, secure with toothpicks.