Step 1Put the water in a small bowl and sprinkle the gelatin over the top. Set aside until the gelatin is fully moistened , at least 3 minutes.
Step 2In a small, heavy-bottomed saucepan, stir together the pumpkin, one-half cup of the sugar, the ginger, cinnamon and nutmeg, and the salt. Over medium heat, bring the mixture to a sputtering simmer, stirring constantly. Reduce the heat to low and continue to cook until the mixture is thick and shiny, 3 to 5 minutes.
Step 3Scrape the mixture into a food processor fitted with the metal blade and process for 1 minute. With the motor running, add the milk, processing until incorporated. Add the egg yolks one at a time, processing just to incorporate, about 5 seconds after each addition. Add the gelatin mixture and pulse in.
Step 4Return the mixture to the saucepan and heat over medium heat, stirring constantly, until thickened slightly (160 to 170 degrees), about 3 minutes. Pour the mixture into a medium bowl and set aside.
Step 5Chill the pumpkin custard by placing the bowl over a larger bowl of ice water, with about 1 tablespoon of salt added to the ice water to speed chilling. Stir occasionally for the first 10 minutes, then slowly but frequently for about 10 minutes longer.
Step 6No more than 20 minutes before you put the pies together, make the meringue: In a mixing bowl, beat the egg whites over low speed until foamy. Add the cream of tartar and beat at high speed until soft peaks form when the beater is raised slowly. Gradually beat in the remaining one-fourth cup sugar, beating until stiff peaks form when the beater is raised.
Step 7When a small amount of the custard dropped from a spoon mounds very slightly on the surface before disappearing, immediately remove the bowl from the water bath and, using a whisk, fold in the meringue just until blended. You will have about 5 cups filling.
Step 8Place the filling in a pastry bag fitted with a large tip, and fill each pie with a generous one-fourth cup filling . Refrigerate until set, at least 2 hours, before serving. Garnish with a dollop of whipped cream and a dusting of nutmeg.