Preheat the oven to 400°F.
Place the wings, garlic, and the onions in a large roasting pan. Place the pan into the preheated oven and roast for 1½ hours.
After it is done, transfer the wings and the onions to a large pot (at least 6 quart.)
Add the cup of water to the roasting pan and deglaze the pan, scraping up all the bits from the bottom. Add the water and drippings to the pot with the turkey wings. Add 6 cups of the chicken broth, the celery, carrots, and thyme to the pot. Bring to a boil and reduce heat to a simmer and simmer uncovered for 1½ hours.
Remove the wings and save them or discard them.
Strain the broth into a smaller saucepan and discard the vegetables. Skim off any fat.
Whisk the flour wit h the remaining 2 cups of chicken broth. Bring the broth in the saucepan to a gentle boil and whisk in the flour/chicken broth mixture, whisking constantly. Bring to a boil and let it boil for 4-5 minutes to thicken. When it coats the back of a spoon it is done.
Add the butter, pepper, and garlic salt (if using it), stir until the butter melts. If you prefer the gravy a shade darker, add the gravy master and stir until evenly combined.
Place into containers and refrigerate for up to a week or freeze up to a month. Enjoy!