1 (3 ounce) package strawberry-flavored gelatin mix (such as Jell-O®)
1/4 cup boiling water
1 (8 ounce) container whipped topping (such as Cool Whip®)
Preheat the oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking pan.
Combine cake mix, water, eggs, and oil in a large bowl; beat with an electric mixer on low speed until moistened. Increase speed to medium and beat for 2 minutes. Pour batter into the prepared baking pan.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, 24 to 29 minutes. Let cool completely, at least 30 minutes.
Pour cold water into a bowl. Sprinkle gelatin mix on top. Let stand until softened, about 1 minute. Add boiling water and stir constantly until granules are completely dissolved.
Poke holes all over the cake using a fork. Pour gelatin mix into the holes. Refrigerate cake until gelatin is set, up to 6 hours.