In a small bowl, sprinkle the gelatin over cold water and let it stand for about 1 minute to soften.
In a saucepan over medium heat, combine the eggnog, sugar, and nutmeg. Bring to a simmer, stirring constantly.
Add the softened gelatin to the heated eggnog mixture and stir until completely dissolved.
Remove from heat and let the mixture cool slightly before pouring it into the graham cracker pie crust.
Refrigerate the pie for at least 2 hours, or until set.
Before serving, top with whipped cream and a sprinkle of nutmeg, if desired.