Gather all ingredients.

Whisk water and gelatin together in a small bowl, then set aside to bloom for about 5 minutes.

Beat egg yolks in a medium heat-safe bowl until thick. Beat in 1 cup sugar, then pumpkin puree, and evaporated milk until each is fully incorporated.

Add cinnamon, allspice, nutmeg, ginger, and salt; mix until well combined.

Place the bowl over a double boiler and cook over boiling water, whisking constantly, until thick, about 15 to 20 minutes.

Add the gelatin mixture, and whisk until fully dissolved. Remove from the heat and cool until mixture thickens and mounds when dropped by the spoonful, about 15 to 20 minutes.

While the pumpkin mixture is cooling, beat egg whites until soft peaks form. Gradually mix in the remaining 1/4 cup sugar and continue to beat until stiff peaks form.

Fold cooled pumpkin mixture into the egg white mixture.

Transfer filling to the graham cracker crust and chill in the refrigerator until firm, about 1 hour.