Preheat an oven to 350 degrees F (175 degrees C). Grease a 9x13 inch baking dish.
Fill a large pot with lightly salted water and bring to a rolling boil over high heat. Once the water is boiling, stir in the shell pasta, and return to a boil. Cook the pasta uncovered, stirring occasionally, until the pasta has cooked through, but is still firm to the bite, about 13 minutes. Drain well in a colander set in the sink.
Place turkey and stuffing in food processor, and pulse until finely ground and combined. Place the ground turkey and stuffing mixture in the bowl of a stand mixer along with the cream cheese, Parmesan cheese, mayonnaise, and 2 cups of the mozzarella. Mix with paddle attachment on medium-low until well blended.
Spread 1/2 cup of gravy on the bottom of the prepared dish. Stuff the pre-cooked pasta shells with the turkey mixture and place in the dish in tight rows. Top with the remaining gravy and the remaining 2 cups of mozzarella cheese.
Bake, covered, for 45 minutes. Uncover and cook for an additional 10 minutes until top is browned and bubbly. Allow to cool for 5 minutes before serving.