Dissolve orange gelatin in boiling water in a large bowl, stirring for about 3 minutes; mix cold water and orange juice into gelatin. Pour gelatin mixture into a glass serving dish. Stir red grapefruit and mandarin oranges into mixture and refrigerate until set, about 4 hours.

Whisk topping and mayonnaise in a bowl until smoothly combined; serve gelatin salad with a spoonful of dressing over each portion.