In small bowl, pour 1/2 cup of the juice. Sprinkle gelatin over juice to soften. In small saucepan, mix remaining juice and sugar. Heat over medium heat just until the sugar dissolves. Stir in the softened gelatin, whisking to mix well. In medium bowl, whisk the mascarpone and vanilla until softened. Gradually whisk in the juice mixture and the milk until well blended. Lightly spray 6 (8-oz) custard cups or ramekins with cooking spray. Evenly pour grapefruit juice mixture into cups. Refrigerate at least 4 hours. To unmold, run tip of knife around edge of each cup. Invert onto dessert plate. To loosen, lift cup slightly to run tip of knife up one part of panna cotta. If desired, garnish with fresh fruit. Makes 6 servings.