Beat egg yolks in a medium saucepan; add 1/2 cup sugar, pumpkin, milk, and seasonings, stirring well. Cook over low heat, stirring constantly, until thickened and bubbly.
Soften gelatin in cold water; let stand 5 minutes. Add to hot mixture, stirring until gelatin dissolves. Stir in walnuts. Let cool; chill until mixture begins to thicken.
Beat egg whites (at room temperature) until foamy. Gradually add remaining sugar, beating until stiff peaks form. Fold egg whites into chilled pumpkin mixture. Pour mixture into pastry shell. Chill overnight. Garnish with whipped cream, if desired.