For the turkey stock, in a large saucepan combine the reserved neck, heart, and gizzard. Add the onions, celery, carrots, parsley, peppercorns, and baby leaf. Cover with about 6 cups water. Bring to a boil over high heat; reduce heat and let simmer, partially covered, on very low heat for 1 to 2 hours. Remove from heat. Strain through a sieve, discard solids. Set aside.
To finish gravy, once you've removed turkey from the roasting pan, place pan over two burners over moderate heat. Use a spatula to loosen any browned bits clinging to the bottom of pan. Sprinkle on the flour and, using a whisk, mix flour with the juices in pan. Let cook 1 minute, stirring, until paste has come together and is beginning to turn a pale golden color. Pour 3 cups of the turkey stock into the pan and whisk to create a smooth gravy. Let simmer 5 to 10 minutes, stirring frequently, until slightly thickened and flavorful. Thin the gravy by adding additional stock as needed. Season to taste with salt and freshly ground black pepper. Keep gravy warm over low heat, stirring occasionally, until ready to serve.