Make the gastrique: In a small saucepot, bring sugar and vinegar to a boil. Remove from heat and let cool, then stir in pomegranate seeds.
Make the salad: Using a paring knife, remove skin and pith from citrus fruit and slice into ¼-inch rings. Place in a bowl and toss with salt, pepper, and 2 tablespoons of gastrique. Arrange on a plate or shallow bowl and spoon remaining gastrique over fruit.