Preheat oven to 350°F. Butter a 9-by-13-inch baking dish; set aside.

In a large bowl, combine melted butter, graham cracker crumbs, and sugar; stir until well combined. Press graham cracker mixture into bottom of prepared baking dish in an even layer; set aside.

In the bowl of an electric mixer fitted with the whisk attachment, whisk together cream cheese, eggs, and sugar until fluffy.

Spread cheesecake mixture over crust in baking dish. Transfer to oven and bake for 20 minutes. Remove from oven and let cool.

Fill a small bowl or liquid measuring cup with ¼ cup water. Sprinkle gelatin over top and let soften.

In a large heatproof bowl, mix together pumpkin, egg yolks, ½ cup sugar, milk, salt, and cinnamon. Set bowl over (but not touching) simmering water; whisk pumpkin mixture until thickened, 8 to 10 minutes. Remove bowl from heat and whisk in gelatin mixture. Let cool to room temperature.

In the clean bowl of an electric mixer fitted with the whisk attachment, beat egg whites until soft peaks form. Add remaining ¼ cup sugar and continue whisking until medium peaks form. Fold into cooled pumpkin mixture. Spread pumpkin mixture over cheesecake; Cover and transfer to refrigerator until set, at least 8 hours and up to overnight.

In the bowl of an electric mixer fitted with the whisk attachment, whisk together heavy cream, sugar, and vanilla until soft peaks form. Spread over cake or serve cheesecake with a dollop on the side.