1. Arrange grapefruit sections in the bottom of the graham-cracker crust. Set aside. In a medium saucepan combine sugar and cornstarch. Stir in water; cook over medium heat until thick and clear. Add gelatin; stir to dissolve. Let cool slightly. Pour gelatin over grapefruit sections in crust. Refrigerate 2 to 3 hours or until firm. Top each slice with whipped topping, and serve.