Dissolve gelatin completely in boiling water in medium bowl. Stir in cold water. Pour slowly into 8-inch square pan. Refrigerate at least 3 hours or until firm. Cut into 1/2-inch cubes. Halve and split bananas just before serving. Place 2 pieces in 4 dessert dishes. Spoon gelatin cubes evenly over bananas. Garnish with whipped topping and chopped walnuts. Makes 4 servings. Note: Recipe can be doubled using 1 (8-serving size) or 2 (4-serving size) packages of JELL-O Brand Watermelon Flavor Sugar Free Gelatin. Use 13 x 9-inch pan.