Pulse gingersnap cookies in a food processor until you have fine crumbs (about 2 cups). Add sugar and cinnamon and pulse a few times to combine. Pour melted butter over and pulse until mixture is well combined.
Transfer crumb mixture to a 9-inch deep-dish pie pan and use a measuring cup to firmly press the mixture evenly onto bottom and up the sides. Bake 5 minutes. Transfer to wire cooling rack to cool completely.
Meanwhile, sprinkle gelatin over 1/4 cup of cold water in a small bowl and set aside.
In a medium saucepan combine egg yolks, 1/4 cup of the sugar, pumpkin, milk, cinnamon, nutmeg and salt; cook and stir over medium heat until thickened, about 10 minutes. Add softened gelatin and stir to combine; transfer to a large mixing bowl to cool.
In another large mixing bowl, beat egg whites on medium speed until foamy; gradually add remaining 1/2 cup of sugar and continue beating until egg whites hold soft peaks and are thick and glossy. Gently fold whites into filling mixture; pour into baked gingersnap crust and smooth with a rubber spatula. Cover and chill at least 3 hours or overnight. Slice and serve with dollops of lightly sweetened freshly whipped cream if desired.
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Notes
Note that nutrition information does not include the whipped cream.
If you're concerned about consuming the raw egg whites in this meringue, use a Swiss meringue instead. A good recipe can be found at seriouseats.com.