Prepare the key lime filling by combining sweetened condensed milk, fresh key lime juice, and lime zest in a medium bowl. Whisk until smooth and creamy.

Make the graham cracker crust by mixing gluten-free graham cracker crumbs, granulated sugar, and melted butter in another bowl. Stir until evenly coated.

Whip the heavy whipping cream with powdered sugar in a clean bowl using an electric mixer until stiff peaks form.

Layer the parfait by adding a layer of graham cracker mixture, followed by a layer of key lime filling. Repeat until glasses are filled, finishing with graham cracker mixture on top.

Cover the parfaits with plastic wrap and chill in the refrigerator for at least 120 minutes. Garnish with lime slices, mint leaves, and reserved graham cracker crumbs before serving.