Butter a 9x13 inch glass baking dish, and set aside. Combine the sugar, corn syrup, and water in a saucepan over medium-high heat. Stir just to blend, then bring to a boil. Heat to 250 to 265 degrees F (121 to 129 degrees C), or until a small amount of syrup dropped into cold water forms a rigid ball.
Meanwhile, whip egg whites with an electric mixer until foamy. Sprinkle in the gelatin powder, and continue whipping until peaks form when the beaters are turned off and lifted.
When the syrup reaches temperature, pour in a thin stream into the egg white mixture, mixing constantly on low speed. Increase to high speed once the syrup is in, and whip until the candy is stiff and loses its gloss. Fold in nuts, and spread into the prepared pan. Allow to stand until firm, then cut into squares.