Cut off the very top of the grapefruit to expose some of the flesh. Cut into the grapefruit with a sharp knife, loosening the flesh. Scoop out the flesh and set aside.
Combine any leftover juice with grapefruit juice to make 260 grams in total.
In a saucepan, combine grapefruit juice, sugar, lemon peel and mint and cook over low heat. Once it comes to a slow boil, turn off heat and add in powdered gelatin and lemon juice. Once gelatin has dissolved completely, put saucepan over ice water. Stir until mixture starts to thicken.
Transfer fresh grapefruit pieces into the grapefruit skin. Pour in gelatin mixture and refrigerate for 1-2 hours or until firm. Serve with whipped cream if desired.