In a small bowl, sprinkle the gelatin over the cold water. Set aside to soften.
In a medium saucepan, mix egg yolks, 1/2 cup sugar, pumpkin purée, milk, cinnamon, ginger, nutmeg, and salt. Cook over medium heat, stirring constantly, until thickened, about 5 minutes. Do not boil.
Remove the saucepan from heat and stir in the softened gelatin until dissolved. Allow the mixture to cool to room temperature.
In a separate bowl, whip the egg whites until soft peaks form. Then, gradually add 1/2 cup of sugar and continue to beat until stiff peaks form.
Gently fold the cooled pumpkin mixture into the beaten egg whites until well combined.
Pour filling into pre-baked crust. Refrigerate for at least 4 hours until firm.
In a medium bowl, whip the heavy whipping cream with the powdered sugar until soft peaks form.
Spread the whipped cream over the chilled pie before serving.