Preheat oven to 350°F (180°C). Lightly grease an 8-inch pie dish.
In a bowl, combine gingersnap crumbs and brown sugar. Stir in melted butter until evenly moistened.
1 ½ cups gingersnap cookie crumbs, ¼ cup brown sugar, ⅓ cup butter
Press mixture firmly into the bottom and sides of the pie dish. Bake for 10 minutes, then let cool completely before filling.
In a small bowl, sprinkle gelatin over cold water (do not stir). Let sit for 1 minute to bloom.
1 tablespoon unflavored gelatin, ¼ cup cold water
In a saucepan, whisk together sugar, cornstarch, salt, and nutmeg. Gradually add eggnog and whisk until smooth.
⅓ cup sugar, 2 tablespoons cornstarch, ¼ teaspoon salt, ½ teaspoon ground nutmeg, 2 cups eggnog
Cook over medium heat, stirring constantly, until thickened and just beginning to boil.
Add bloomed gelatin and stir until dissolved. Remove from heat and let cool to room temperature. Stir in vanilla and rum extracts. Gently fold in whipped cream until fully incorporated.
1 teaspoon vanilla extract, 1 teaspoon rum extract, ½ cup heavy whipping cream
Pour the filling into the cooled crust. Chill in the refrigerator for at least 3 hours, or until firm.
Garnish with whipped cream and a sprinkle of nutmeg before serving, if desired.