STIR the stock, tomatoes, cheese, Italian seasoning and garlic powder in a shallow nonmetallic dish or a gallon-size resealable plastic bag. Add the chicken and turn to coat. Cover the dish or seal the bag and refrigerate for 30 minutes. Remove the chicken from the marinade.
LIGHTLY oil the grill rack and heat the grill to medium. Grill the chicken for 15 minutes or until it's cooked through, turning the chicken over halfway through cooking and brushing often with the marinade. Discard the remaining marinade.