To make a mango mousse pie with gelatin, shape and chill pie dough in a pie plate, line it with foil, and place it on a low rack of an oven that's preheated to 425 degrees Fahrenheit. After baking, stir gelatin in cool water, and mix mango puree and sugar in a saucepan over low heat. Stir in gelatin and mango puree, cool the mixture in an ice bath, fold in whipped cream, and spread onto the pie shell.
Move the oven rack to a lower position before preheating the oven. After lining chilled dough with foil and putting dried beans as weights, bake the dough for 15 minutes. Remove the liner and weights, rotate the pie, bake for another 10 minutes, and let the pie shell cool.
Create an ice bath the size of a small saucepan, bloom 2 1/2-teaspoons powdered gelatin in 1-ounce cold water, and leave the gelatin for five minutes. Prepare 13-ounces strained mango puree. Stir in half the puree and 5-ounces granulated sugar in a saucepan. Stir in gelatin, set aside the mixture, add in the remaining mango puree, and stir well while the mixture rests on the ice bath for cooling.
After folding in 12 ounces chilled heavy whipped cream, add the mousse to the pie shell, and refrigerate for two hours or more. Smooth the top with the remaining whipped cream, and serve.