Dissolve the unflavored gelatin in 2 ½ cups cold water in a bowl.

Combine the fruit-flavored Jell-O, sugar, and hot water in a large saucepan; bring to a boil. Stir the unflavored gelatin mixture into the boiling mixture; cook and stir until the gelatin and sugar are completely dissolved, about 5 minutes.

Remove the saucepan from heat. Stir 1 cup cold water into the gelatin mixture; pour into a 13x9-inch dish.