Pour 1/4 cup cold water into a bowl; sprinkle gelatin on top. Let stand 10 minutes. In a pan over medium heat, mix cream and sugar. Cook just until mixture begins to bubble. Add gelatin; stir until dissolved. Stir in buttermilk and lemon juice. Divide custard among 6 6-oz. glasses. Cover with plastic wrap; chill for at least 8 hours.
Make salad: Cut off top and bottom of grapefruit. Slice off peel and white pith. Working over a bowl, cut in between membranes to extract segments. Repeat with oranges. Add sugar and mint. Let stand 10 minutes; drain. Uncover panna cotta, top with citrus salad; serve.