Preheat your oven to 350°F (175°C). In a medium bowl, combine graham cracker crumbs, sugar, and salt. Pour in melted butter and mix until the texture resembles wet sand. Press the mixture firmly into a 9-inch pie plate, creating an even layer along the bottom and up the sides.
Bake the crust for 8-10 minutes until lightly golden. Allow to cool completely before filling.
In a small bowl, sprinkle the gelatin over cold water and let stand for 5 minutes to soften.
Combine pumpkin puree, milk, egg yolks, ½ cup sugar, salt, and pumpkin pie spice in a medium saucepan. Cook over medium heat, stirring constantly, until the mixture thickens enough to coat the back of a spoon (about 5-7 minutes). Remove from heat.
Add the softened gelatin to the hot pumpkin mixture and stir until completely dissolved. Stir in the vanilla extract. Transfer to a large bowl and allow to cool to room temperature, stirring occasionally to prevent a skin from forming (about 15-20 minutes).
In a clean, dry bowl, beat the egg whites with cream of tartar until foamy. Gradually add the remaining ¼ cup sugar and continue beating until stiff, glossy peaks form.
When the pumpkin mixture has cooled but not set, gently fold in the whipped egg whites in three additions. Use a rubber spatula and a light hand to maintain as much air as possible in the mixture.
Pour the filling into the prepared crust, smooth the top, and refrigerate for at least 4 hours or overnight until firmly set.
Just before serving, whip the heavy cream with powdered sugar and vanilla until soft peaks form. Spread or pipe the whipped cream over the chilled pie and dust lightly with cinnamon.