In a large mixing bowl, combine cake mix, flour, eggs, oil, water, gelatin, and about 3/4 of the thawed strawberries. Pour batter into two greased and floured 9-inch layer cake pans or 9x13x2-inch baking pan. Bake at 350° for 30 to 35 minutes. Cool in pans for 10 minutes; remove layers to rack to cool completely. 9x13x2-inch cake may be left in pan for frosting and serving.
In a medium bowl, mix confectioners' sugar, butter, and remaining strawberries until smooth and creamy (leave some of the excess strawberry juice out and add back a little at a time, so frosting isn't too thin). Frost cooled cake; store in refrigerator.