Stir together graham cracker crumbs, melted butter, and sugar until the texture of wet sand. Press firmly into the bottom and 1 inch up the sides of a 9-inch springform pan. Bake at 350F for 8 minutes until fragrant and just set. Cool completely on a wire rack, about 20 minutes.

Puree diced mango in a blender or food processor until completely smooth, about 1 minute. You need 1.5 cups puree; reserve any extra for garnish. Set aside.

Sprinkle gelatin over cold water in a small bowl. Let stand 5 minutes until softened. Microwave for 15 seconds until liquid and warm but not hot, stirring to dissolve completely. Set aside to cool slightly, about 3 minutes.

Beat cold heavy cream in a large bowl with an electric mixer on medium-high speed until stiff peaks form, about 3 minutes. Transfer to a separate bowl and refrigerate.

Beat cream cheese in the same large bowl until smooth and fluffy, about 2 minutes, scraping down sides twice. Add sweetened condensed milk, key lime juice, key lime zest, and mango puree. Beat on medium speed until fully combined and silky, about 1 minute.

With mixer on low, slowly drizzle in the warm gelatin mixture until incorporated. Immediately fold in the whipped cream in three additions, cutting through the center and turning the bowl each time, until no streaks remain. Pour into cooled crust and smooth the top with an offset spatula.

Refrigerate uncovered until completely set, at least 5 hours or preferably overnight. Run a thin knife around the edge before releasing the springform. Garnish with reserved mango puree, fresh mango slices, or lime wheels if desired.