Heat oil in large nonstick skillet over medium-high heat. Sprinkle chicken on each side with garlic salt and pepper. Place in skillet; cook 6 minutes to brown, turning once. Remove from skillet; keep warm.
Place zucchini and onion in skillet. Cook 5 minutes or until tender, stirring occasionally. Stir in undrained tomatoes.
Return chicken to skillet. Cover; reduce heat to medium-low and simmer 10 to 12 minutes or until chicken is no longer pink in centers (165°F). Serve vegetable mixture over chicken.