Prepare grill with hot coals around sides. Place foil drip pan in center.
Remove zest from orange with vegetable peeler; avoid bitter white pith. Finely chop zest. Halve orange; juice orange halves; reserve juice for gravy; reserve orange halves.
Combine zest, sugar, poultry seasoning and salt in bowl. Finely chop white part of green onions; reserve green parts. Add chopped whites and chopped parsley to zest mixture.
With fingers, carefully loosen skin around turkey breast and legs. Spread zest-herb mixture evenly under skin. Place parsley stems, green onion ends and orange halves in turkey cavity.
Place neck and gizzard in drip pan. Position rack 6 inches above coals. Place turkey on grill rack over drip pan.
Grill, covered, for 3 hours or until thermometer inserted in thigh registers 175 degrees F. Add briquettes to each side of grill every 50 to 60 minutes to maintain heat. Let the turkey stand for 20 minutes before carving.
Discard the neck and gizzard from drip pan. Pour drippings into liquid measure. Skim off fat. Add water to equal 4 cups. Pour into saucepan along with reserved orange juice. Bring to simmering. Whisk together milk, flour, salt and poultry seasoning in bowl until smooth. Stir into liquid in saucepan. Simmer, stirring, 4 minutes or until thickened. Serve gravy with sliced turkey. Makes 24 servings.
Heat oven to 350 degrees F. Roast turkey on rack in large roasting pan, loosely covered with foil, in a 350 degree oven for 3-1/2 hours or until thermometer inserted in thigh registers 175 degrees F.