Arrange rack in center of oven and preheat to 425°F. In roasting pan, toss bones with 1/4 cup oil. Roast until well browned, about 20 minutes. Remove from oven and set aside. In heavy, large pot over moderately low heat, heat remaining 1/4 cup oil until hot but not smoking. Add garlic, onions, celery, and carrots and saute until golden brown, 16 to 18 minutes. Add peppercorns and continue cooking until aromatic, 2 to 3 minutes. Add brandy and simmer, uncovered, until reduced by half, about 1 minute. Add turkey bones, chicken stock, and demi-glace and bring to a boil. Reduce heat to low and simmer until thickened and reduced by half, 2 to 2 1/2 hours. Strain, return to pot, and place over moderately low heat. Skim excess fat, then add basil, remove from heat, cover, and let steep 10 minutes. Strain gravy through cheesecloth-lined sieve into large pot. Stir in salt and keep warm.