For the Gravy:
2 to 3 cups low-sodium chicken broth
4 tablespoons unsalted butter
cup all-purpose flour
Kosher salt and freshly ground pepper
Brine the turkey:
Rinse the turkey parts and place in doubled 21/2-gallon resealable plastic bags (or a large stockpot). Add 1/4 cup salt, the honey, garlic, bay leaves, thyme, 2 sprigs sage, the peppercorns, allspice and celery leaves. Add enough cold water to cover the turkey (about 3 quarts). Press out the air, close the bags and place in a large bowl or other container to protect against any leaks. Refrigerate at least 6 hours or overnight. Roast the turkey: Position racks in the upper and lower thirds of the oven and preheat to 425.
Remove the turkey parts from the brine and pat dry with paper towels. Place skin-side up on racks set in 2 medium roasting pans (be sure to leave space between the turkey parts to allow air to circulate). Pour 1 cup water into each pan. Drizzle the turkey parts with melted butter and scatter with the leaves from the remaining 2 sage sprigs. Place 1 pan on each oven rack and roast until the turkey begins to brown, about 30 minutes.
Lower the oven temperature to 400. Switch the positions of the pans and rotate from front to back. Continue roasting until a thermometer inserted into the thickest part of the breast (avoiding bone) registers 165degrees, 1 hour to 1 hour, 15 minutes. Transfer the turkey parts to a platter and tent with foil.
Make the gravy: Pour the pan juices into a 4-cup glass measuring cup and let rest until the fat rises to the top, 2 to 3 minutes. Skim off and reserve the fat. Set 1 roasting pan across 2 burners, add 2 cups chicken broth and bring to a simmer over medium-high heat, scraping up any browned bits. Add the simmering broth to the pan juices in the measuring cup, then add the additional chicken broth, if needed, to equal 4 cups liquid.
Melt the butter in a medium saucepan over medium-low heat. Whisk in the flour and cook, whisking constantly, until smooth, about 2 minutes. Gradually whisk in the broth mixture and any collected juices from the platter. Increase the heat to medium high and boil, uncovered, until thickened, about 8 minutes. Season with salt and pepper.
Carve the turkey pieces and garnish with sage. Serve with the gravy.