On a lightly floured surface, roll out the crust to a 14-inch round. (If using store-bought crusts, brush 1 with water; top with second for double thickness before rolling out.) Transfer to a parchment-lined baking sheet (it will stretch out over the edge of the parchment). Fold about 1/3 inch of the edge inward all around; brush off flour. Fold over 3 more times, brushing lightly with cream and sprinkling lightly with sugar between each fold. Pinch the folded edge in slightly to form a 1-inch-high, 3/4-inch-wide rim. Pierce the bottom all over with a fork. Freeze 1 hour.
Preheat the oven to 475°. Bake the crust 8 minutes. Cover the bottom of the crust with a round piece of foil. Reduce the oven temperature to 375°; bake until edges are golden brown, piercing the bottom with a fork if bubbles form, 15 to 20 minutes more. Discard foil. Let cool on a rack.
Pour the Grand Marnier into a small bowl. Sprinkle with the gelatin; let stand until gelatin softens, about 10 minutes. In a large metal bowl, combine 1 cup cream, 1/2 cup sugar, the pumpkin, egg yolks, pie spice and salt. Set over a large saucepan of simmering water; stir until the mixture registers 160° on an instant-read thermometer, about 5 minutes. Add the gelatin mixture; stir until dissolved, about 1 minute. Set the bowl over a large bowl of ice water. Stir until cooled to room temperature, about 5 minutes.
Using an electric mixer, beat the egg whites until foamy. Gradually beat in 1/4 cup sugar until soft peaks form. Adding half at a time, fold egg whites into pumpkin mixture. Set the bowl over the bowl of ice water; let cool, folding occasionally, until the filling holds its shape like firmly whipped cream, about 1 hour. Spoon into the center of the crust. Refrigerate until set, about 2 hours, or covered up to 1 day.
In a skillet, cook the pepitas and 3 tbsp. sugar over medium heat until caramelized, about 5 minutes. Transfer to a plate to cool, then break apart into small clumps.
Just before serving, beat 3/4 cup cream until soft peaks form; spread over the pie. Sprinkle with the candied pepitas.