Preheat oven to 400 degrees F. In a large roasting pan, toss the chopped onion, carrots and celery with olive oil, season with salt and pepper.
Place turkey parts on top of vegetables. Brush with olive oil and season with salt and pepper. Roast for about 90 minutes, until turkey is cooked through and browned.
Transfer turkey and vegetables from the roasting pan to a large stock pot. At this point you can remove some meat from the bones and set aside for another use, but leave some on for the stock.
Place roasting pan over two burners on medium heat. Add the wine and deglaze the pan - stir to scrape up the browned bits on the bottom of the pan. If you need more liquid, use some chicken stock. When bottom of pan is clean, pour liquid into stock pot with the turkey parts and vegetables.
To the stock pot, add the chicken stock and water, as well as the rosemary, thyme and bay leaf. Bring to a boil, then reduce heat and simmer for several hours. Check frequently and add more water if needed.
When done simmering, strain stock through a fine mesh sieve, discarding solids.
At this point you can refrigerate your sock if you have the time. This will make the fat separate so you can easily remove it to use for the roux. If you don't have time, use a fat separator or allow fat to rise to the surface and skim it off.
When ready to make gravy, make sure the turkey stock is hot. If you have refrigerated it, reheat it before using.
Add 1/2 cup of fat to a sauce pan (if you don't have enough turkey fat, you can make up with difference with butter). Sprinkle on the flour and stir together, cooking for about two minutes.
Slowly whisk in 1/2 cup of hot stock, whisking to remove any lumps. Continue to add stock a little at a time, whisking constantly. When all 4 cups are added, bring gravy to a simmer, whisking constantly, until thickened. If it is too thick you can add more stock until desired consistency is reached. Taste and add more salt and pepper if necessary.
At this point, gravy can be served right away or stored and refrigerated or frozen.
To store gravy - allow to cool slightly, then transfer to a freezer safe container. Store in the refrigerator for up to 4 days, and in the freezer for up to 4 months.
To reheat gravy - Thaw frozen gravy in the refrigerator for one to two days. Reheat in a sauce pan over medium heat, whisking until hot. If needed, you can add more hot chicken or turkey stock to thin gravy to desired consistency.