For the crust: Preheat oven to 375°. Coat two 9" pie plates with a thin film of the butter and set aside. Put gingersnap crumbs, sugar, and nutmeg into a large mixing bowl and stir to combine. Add the remaining butter and mix well. Divide crumb mixture between prepared pie plates. Spread crumbs evenly on bottom and up sides of pie plates, then, using your fingertips, firmly press down on crumbs to form a tightly packed crust. Bake crusts until lightly browned, about 8 minutes. Remove crusts from oven and set on wire racks to cool completely.
For the filling: Put pumpkin, sugar, salt, cinnamon, ginger, nutmeg, and cloves into a medium heavy-bottomed saucepan and heat over medium heat, stirring often, until sugar dissolves, about 5 minutes. Meanwhile, mix gelatin and 1⁄4 cup cold water together in a large bowl and set aside until gelatin swells and softens, about 5 minutes. Bring 1 cup water to a boil in a small pot over high heat, then add to gelatin, stirring until gelatin completely dissolves, about 2 minutes. Add pumpkin mixture and mix well. Set aside to cool to room temperature.
Beat cream in another large bowl with an electric mixer on medium speed until soft peaks form, 1–2 minutes. Gently fold one-third of the cream at a time into pumpkin mixture. Divide filling between prepared crusts. Cover pies with plastic wrap, being careful not to let it touch surface of filling, and refrigerate until well chilled and firm to the touch, about 40 minutes. Garnish pies with walnuts and serve immediately.