In a small bowl, add gelatin and water, stir, and set aside for at least 5 minutes.

Cut grapefruits in halves horizontally, and squeeze juice out without breaking or crushing the fruits' skins, keeping their shape. Save both juice and skins. Then remove the remaining grapefruit from inside the grapefruit skin forming an empty cup, and clean the inner surface. Cut thinly on the bottom of the skin if it does not sit well without tipping or rolling.

Combine grapefruit juice, water, sugar, and lemon juice in a pot, and heat to very warm, until the sugar has dissolved. Remove from heat, add soft gelatin, and stir until the gelatin has completely dissolved.

Cool the pot in an ice water bath until the liquid gets slightly thick, stirring constantly. Pour the mixture into prepared grapefruit cups (4-5 cups).

Chill in the fridge at least 2 hours or overnight. Garnish with mint leaves before serving.