Pour the orange juice into a heatproof bowl.
Sprinkle the gelatin over the juice and set aside.
Peel mango, cut out the pits, and purée in a blender or food processor.
Pour this purée (approximately 4 cups) into a mixing bowl. Stir in sugar and milk.
Place the bowl that contains the juice and gelatin over a pan of warm water and mix ingredients until smooth. Whisk the mixture into the mango combination.
Stir in whipped topping. Rinse a mold with cold water and pour in the mixture.
Refrigerate, covered, until it gels (about 3 hours). Remove from the mold when ready to serve. Serve cold with fresh fruit.