In a small bowl combine garlic, marjoram, salt, and pepper; set aside. For glaze, in a small saucepan stir together the orange juice and cornstarch; add half of the seasoning mixture. Cook and stir over medium heat until thickened and bubbly. Cook and stir 1 minute more. Stir in maple syrup and orange peel; set aside.
Brush the turkey roast with melted butter and rub with remaining seasoning mixture. Insert a meat thermometer near the center of the roast.
For a charcoal grill, arrange medium-hot coals around a drip pan. Test for medium heat above the pan. Place the turkey roast directly on the grill rack over the pan. Cover; grill for 2 to 2-1/2 hours or until thermometer registers 165F. Add coals to maintain heat every 30 minutes. Brush with glaze the last 30 minutes of grilling. Remove meat from grill. Cover with foil; let stand for 15 minutes before carving. (The meats temperature will rise 5F during standing.) Bring any remaining glaze to boiling; pass with turkey. (For a gas grill, preheat grill. Reduce heat to medium. Adjust for indirect cooking. Grill as above.)