In a small saucepan, sprinkle gelatin over the cold water, and let soften 5 minutes. Cook softened gelatin over medium heat, swirling pan, just until gelatin is dissolved; do not let boil. Let cool completely.
Place pumpkin puree in a large bowl. Stir in softened gelatin, then add egg yolks, maple syrup, nutmeg, vanilla, ginger, allspice, salt, white pepper, and rum. Whisk until fully blended.
With an electric mixer on medium speed, whisk egg whites and the sugar to soft peaks. Gently fold egg-white mixture into pumpkin mixture to combine. Whip heavy cream on medium high to stiff peaks, then gently but thoroughly fold into pumpkin mixture.
Divide mousse among 8 to 10 glasses; refrigerate until set, at least 2 hours, or up to 1 day, covered with plastic wrap. If desired, top each with a dollop of sweetened whipped cream and a pastry leaf before serving.
Food Safety Note: The eggs in this recipe are not cooked.