In a small saucepan, bring sugar and water to a boil; cook and stir until sugar is dissolved. Remove from heat; cool slightly., Stir in grapefruit juice. Transfer to an 8-in. square dish. Freeze 1 hour. Stir with a fork. Freeze 2-3 hours longer or until completely frozen, stirring every 30 minutes., Cut a thin slice from the top and bottom of each orange. Stand oranges upright on a cutting board. With a knife, cut off peel and outer membrane from oranges. Working over a bowl to catch juices, cut along the membrane of each segment to remove fruit. Place fruit in bowl. Repeat with grapefruit, removing any seeds., Peel and separate clementines into segments; add to oranges and grapefruit. Gently stir in pomegranate seeds., To serve, stir granita with a fork. Alternately layer granita and fruit mixture into six dessert dishes. Serve immediately.