In a small saucepan, add milk. Sprinkle gelatin over the top and let sit 5 minutes without stirring. The mixture will be very thick. Add remaining 2 tablespoons milk and stir to thin.
Transfer saucepan to a stove and heat over medium-low flame. Slowly heat the gelatin mixture, stirring frequently, until the gelatin is completely dissolved, about 3 minutes. Remove from heat and cool to room temperature, at least 15 minutes.
While the gelatin mixture is cooling, in a mixing bowl with an electric hand mixer, beat the cream cheese until smooth. Beat in sugar, vanilla, pumpkin, and pumpkin pie spice until smooth.
Beat in cooled gelatin mixture until uniformly combined. Scrape into graham cracker crust and smooth the top. Chill until firm, at least 4 hours or overnight. Serve with whipped cream and cinnamon sugar topping if desired.
Gelatin: Look for envelopes (not sheets) such as Knox brand unflavored gelatin. The two types of gelatin are usually interchangeable, but this recipe hasn’t been tested with gelatin sheets so I don’t know for sure.
Pumpkin pie spice: Or substitute 1 teaspoon cinnamon, ¼ teaspoon ginger, and ¼ teaspoon nutmeg.