Blind-bake the pie crust in a 400 degree F. oven for 15 minutes with pie weights, then another 15 minutes until fully baked. Cool completely.
Dissolve the gelatin by sprinkling it over the water in a small bowl and let sit for 5 minutes.
In a medium saucepan, combine ½ cup of the sugar with the salt, cinnamon, allspice, nutmeg, cloves, egg yolks, milk, and pumpkin. Whisk well and cook, stirring frequently, for 5-7 minutes, just until the mixture is hot and has thickened slightly, but hasn't come to a boil.
Remove from the heat and stir in the dissolved gelatin and vanilla. Cool completely to room temperature but don't refrigerate yet since the gelatin will set in the fridge.
When the filling has cooled, beat the egg whites until soft peaks start to form. Continue beating, gradually adding the remaining ¼ cup of sugar until stiff peaks form.
Gently fold the stiffened egg whites into the pumpkin mixture, then pour it into the cooled pie shell. Refrigerate at least 4 hours or overnight until firm, then top with sweetened whipped cream and dust with cinnamon for presentation.