I have actually done this. When we hosted Thanksgiving. (DxW has a large family). I say a gallon, Plus for us, some extra for leftovers. We make gravy the day before. Roast/ brown/boil up turkey wings, backs, necks and make a rich stock . ( I've used a couple chopped up thighs when I couldn't get enough inexpensive parts). Make gravy. Cool. Store overnight in a 5 quart ice cream bucket. Make 2 turkeys for thanksgiving. Combine the chilled gravy with the drippings from both roasting pans , Thin or thicken as needed. Voila, there is plenty of gravy.
The premade gravy nearly fills the ice cream bucket, so there's a gallon. Depending on, there is enough leftover for several regular meals.
I looked up 1 gallon makes about 16 cups. CWS is about right too. 15 cups @ 1/3 cup serving size. I think both estimates are good comparisons.