Prepare your smoker or grill with a smoke box according to manufacturer's directions, using apple or cherry wood chips or chunks and preheat it to 250°F (120°C). Get a good stream of smoke going before adding the turkey necks.
Place the turkey necks in the smoker and cook until they reach an internal temperature of 165°F (72°C), about 2 ½ hours. Replenish the wood as needed to maintain a steady stream of smoke.
Transfer the cooked turkey necks to a large pot and add the onion, carrot, celery, parsley, peppercorns, bay leaf, and water. Bring to a simmer and cook until reduced by half, about 1 ½ hours.
Let the stock cool slightly, then strain it through a fine mesh sieve, discarding the turkey necks and vegetables. If you're not using the turkey broth immediately, let it cool to room temperature, then store it in the fridge for up to 1 week. In a medium saucepan, melt the butter. Add the flour and cook, stirring constantly, until you have a thick roux, about 1 minute.
If using turkey drippings, slowly add up to ½ cup of drippings to the roux, whisking constantly. If not, proceed to the next step.
Slowly add 2 cups of the turkey broth to the gravy, whisking constantly. Let it come to a simmer and cook until it begins to thicken, about 2 minutes. Continue adding broth until your reach your desired consistency. You may not need all of the broth.
Season to taste with salt and pepper. Serve warm.