Soften gelatin in cold water; set aside.
Beat egg yolks until thick and lemon colored. Combine yolks, 1 cup sugar, lemon rind and juice, and salt in a saucepan; stir until smooth. Cook over medium heat, stirring constantly, 5 minutes or until thickened. Remove from heat; add gelatin. Stir well. Cool.
Beat egg whites (at room temperature) until foamy; gradually add remaining 3/4 cup sugar, 1 tablespoon at a time, beating until soft peaks form.
Fold egg whites into lemon mixture; pour into piecrust. Chill. Garnish with whipped cream and lemon, if desired.