Crush crackers to crumbs and mix with melted butter.
Press 2/3 of crumb mixture over bottom of a 9-inch Springform pan and chill. Cut off peel and pith from grapefruits, holding over a bowl to catch juice. Cut out sections from between membranes and set aside. Squeeze membranes into bowl to extract juice. Sprinkle gelatin over juice and let stand 2 to 3 minutes, until softened. set bowl of gelatin in a pan of hot water and stir until dissolved. In a bowl, beat cream cheese, half and half and sugar. Stir in gelatin and lemon zest and juice.
In a large bowl, whisk egg whites until stiff. Fold into creamy mixture. Pour over crust and refrigerate until set. To serve, remove cheesecake from pan and decorate with reserved grapefruit sections. Press remaining crumbs evenly into sides of cheesecake.