Cut a thin slice from the bottom of each grapefruit half so that it sits flat. Squeeze grapefruit halves, reserving 3 cups pulp and juice. Gently remove membrane, leaving shells intact; scallop edges, if desired. Freeze shells.
Combine sugar and water in a heavy saucepan. Cook over medium heat, stirring frequently, until mixture comes to a boil and sugar dissolves. Cook, stirring frequently, until mixture reaches soft ball stage (240°). Set aside.
Combine egg whites (at room temperature), cream of tartar, and salt in a large metal mixing bowl; beat until stiff peaks form. Slowly pour hot syrup in a thin stream over egg whites while beating at medium speed of an electric mixer. Turn mixer to high speed, and continue beating 5 minutes or until stiff. Add reserved 3 cups grapefruit pulp and juice, stirring well. Freeze mixture 4 hours.
Remove from freezer, and stir grape fruit mixture until well blended. Repeat freezing procedure until mixture reaches the consistency of a sherbet. Spoon into prepared grapefruit shells. Freeze until firm.
Remove from freezer 15 minutes before serving. Place on individual serving plates; garnish each with a cherry, if desired.