Cut grapefruit in half; use a sharp knife to separate sections of grapefruit from the peel.
Combine sugar and cinnamon.
Remove 1/4 tsp. of the cinnamon mixture; sprinkle remaining cinnamon mixture over cut sides of grapefruit.
Place, cut sides up, in broiler pan.
Broil, 3 to 4 inches from heat, 3 to 4 minutes or until hot and bubbly. Top each with 2 Tbsp. of the whipped topping; sprinkle with the reserved cinnamon mixture.