Remove giblets from chicken. Reserve for another use. Rinse and drain chicken; pat dry.
Place onion in cavity of chicken. Brush chicken on all sides with some of the teriyaki sauce. Sprinkle with garlic-pepper seasoning. Place chicken, breast side up, on a rack in a roasting pan coated with cooking spray. Insert meat thermometer into meaty part of thigh, making sure it does not touch the bone. Pour remaining teriyaki sauce over chicken.
Bake, uncovered, at 375° for 1 1/2 hours or until meat thermometer registers 185°. Remove skin before serving.