1Heat oven to 350°F (325°F for dark or nonstick pan). Spray bottom only of 13x9-inch pan with cooking spray.
2Make and bake cake mix as directed on box for 13x9-inch pan, using water, oil and egg whites and adding 2 teaspoons of the lemon peel. Cool in pan on cooling rack 20 minutes.
3Meanwhile, in small bowl, pour boiling water over gelatin; stir until gelatin is dissolved. Poke cake every inch with handle of wooden spoon or tines of meat fork halfway into cake. Pour gelatin slowly over cake to fill holes. Cool completely, about 1 hour longer.
4In small bowl, mix frosting, lime peel and remaining 1 teaspoon lemon peel until well blended. Frost cake. Sprinkle with sugar.