Heat the oven to 425 degrees. Combine the turkey, celery, carrots, garlic and thyme in a large roasting pan. Toss with the olive oil and roast, stirring occasionally, until the meat is browned and the vegetables are caramelized, about 1 hour.
Transfer the contents of the pan to a large soup pot. Add the bay leaf, onion, chicken broth and cold water; bring to a boil. Meanwhile, place the roasting pan over medium heat and add the wine. Bring to a simmer, scraping up the browned bits with a spatula; pour liquid into the pot. Reduce heat and simmer gently until the stock is reduced to 5 cups, about 1-1/2 hours.
Strain the stock into a large bowl, pressing on the vegetables to extract as much liquid as possible. Refrigerate until the fat rises to the surface and congeals, about 2 hours. (The stock can be made up to two days in advance and kept refrigerated until ready to use. Or, make the stock, chill completely, and freeze for up to two months.)
Skim the fat off the stock and transfer 6 tablespoons of the fat to a medium saucepan set over medium heat. When the fat is sizzling, add the flour and whisk constantly for 1-1/2 minutes. Gradually whisk in the turkey stock, a little at a time, letting the roux fully absorb the liquid before adding more (don't worry that the stock is cold and spoonable; it will instantly liquefy once it hits the hot pan). Bring gravy to a simmer and cook until thickened and bubbly, 20 minutes. Season with salt and pepper. Cool completely, cover and refrigerate for up to two days or freeze for up to three months (let it thaw in the refrigerator for one day).
On the big day: Reheat the gravy in a saucepan set over medium heat until simmering. You can add up to 1 cup of the pan drippings from your roasted turkey to the gravy. To do so, strain the pan drippings into a glass measuring cup, skim off the clear fat that rises to the surface, and discard. Whisk 1 cup of the drippings into the gravy and simmer until reduced slightly, about 10 minutes. (If you prefer thicker gravy, make a slurry by mixing a few tablespoons of flour with an equal amount of cold water. Add to the gravy and simmer until thickened, 20 minutes.)