Place the wings, celery, onion and garlic in a roasting pan. Place in oven and roast for 2 hours.
Place roasting pan on a burner on the stove (if you don't have room, you can transfer the contents to a large stockpot).
Add 8 cups of water, the peppercorns and bay leaves. Bring to a boil, then reduce to a simmer and cook for 1 hour.
Remove turkey and vegetables. Strain the resulting stock into a large resealable container.
Refrigerate for at least 8 hours. Skim off any fat.
For the gravy
Melt butter in a large pot or Dutch oven over medium-high heat.
Whisk in the flour. Cook, whisking constantly, until golden in color.
Slowly pour in 2-3 cups of the stock, a little at a time, whisking constantly.
Add in the vinegar. Season with salt and pepper, to taste.
Whisk in some of the milk. Taste and add more if you wish, but don't make the gravy too thin. Let simmer until the desired thickness is achieved. Add more milk if it gets too thick.