Make crust: Preheat oven to 325 degrees. In a food processor, combine graham crackers, sugar, and salt; process until fine crumbs form. With machine running, slowly pour butter through feed tube and process until mixture resembles wet sand. Press crumbs in bottom and up sides of a 9-inch square baking dish. Bake until crust is deep golden brown and fragrant, 20 minutes. Let cool completely in baking dish on a wire rack.
Meanwhile, make filling: Place 1/4 cup cold water in a small bowl. Sprinkle gelatin on top and let stand 5 minutes. In a large bowl, whisk together pumpkin puree and cream cheese until smooth. In a small saucepan, combine evaporated milk, brown sugar, cinnamon, nutmeg, and salt; bring to a simmer over medium. Add gelatin mixture and stir until completely dissolved. Pour milk mixture into pumpkin mixture and whisk until completely smooth.
Pour filling into cooled crust and refrigerate until chilled and completely set, about 3 hours (or wrap tightly with plastic and refrigerate, up to 2 days). To serve, whip cream with confectioners' sugar until soft peaks form. Top pie with whipped cream and sprinkle with nutmeg.