Remove your turkey and rack from the roasting pan to allow it to rest.
Dump all the vegetables and drippings from the pan into a giant measuring cup (8-12 cup capacity).
The turkey fat will float to the top. Scoop out about 1/2 to two thirds of that. Remove scoops of vegetables until you have a total of 8 cups of broth, fat, and veggies.
Grind half of the broth/veg mixture at a time in your food processor. Process 3-4 minutes per batch until it is the smoothness you desire. Usually I add the veggies and just some of the broth which makes it easier.
Put the ground veggies and any broth you set aside into a 12" skillet over medium heat. Whisk in flour 2 Tbsp. at a time. Be careful to mix it all the way in so you don't end up with lumps. If you find any large chunks of vegetables remove and discard.
Add the black pepper, white pepper, garlic powder and butter. Continue to whisk frequently and then constantly until the gravy comes to a simmer.
Maintain the simmer, adjusting heat as necessary, for 5-10 minutes. Whisk constantly. The gravy will thicken.
Remove from heat and add 1/2 cup of heavy cream. Taste and add the salt if desired. Serve hot.