In a large mixing bowl, combine the graham cracker crumbs, sugar, and melted butter. Stir until the mixture resembles wet sand. Press the mixture firmly into the bottom and sides of a 9-inch pie plate. Bake in a preheated oven at 350°F (175°C) for 10 minutes or until lightly golden. Remove from the oven and let it cool completely.

In a medium bowl, combine the lime gelatin with one cup of boiling water. Stir until fully dissolved, which should take about 2-3 minutes.

Once the gelatin is dissolved, mix in one cup of cold water. Allow the mixture to cool at room temperature for about 15 minutes.

In a separate mixing bowl, combine the sweetened condensed milk, freshly squeezed lime juice, and lime zest (if using). Stir until fully incorporated.

Slowly pour the cooled gelatin mixture into the bowl with the sweetened condensed milk. Fold the mixtures together gently until smooth and well combined.

Pour the lime filling into the cooled graham cracker crust. Spread it evenly, ensuring no air bubbles remain.

Cover the pie with plastic wrap or aluminum foil and refrigerate for at least 4 hours, or until set. For best results, overnight refrigeration is recommended.

Once set, remove the pie from the refrigerator. Top with whipped cream and garnish with lime slices or wedges. Slice and serve chilled.