Last year I had to do turkey gravy for 200 people. I bought 25 pounds of turkey necks, chopped them up with a cleaver, and made a rich turkey stock in a 16 gallon pot. Then I made a lot of roux and made gravy. Worked great! To make stock, I browned up the neck parts in the oven, then covered them with cold water, added half an onion (no need to peel it) plus some diced carrot and celery and probably a cheesecloth bag of herbs/spices too. See Joy Of Cooking or your favorite cookbook for how to make chicken stock.
By the way I made *way* too much gravy! Next time I'd make maybe 8 gallons for 200 people!