Preheat oven to 350 degrees F. Grease and flour a 9-inch x 13-inch baking dish. Set aside.
Prepare the cake mix with water, vegetable oil, and egg whites according to package directions.
Pour the batter into the prepared dish and bake according to the package directions for a 9x13 pan. Remove the cake from the oven and let cool to room temperature, about 1 hour.
After the cake has cooled, poke holes all over the cake with a fork or the back of a thin wooden spoon. Set aside.
Bring 1 cup of water to a boil in a medium saucepan. Remove from the heat. Pour in the gelatin mix and whisk for about 2 minutes until well dissolved. Add 1/2 cup of cold water and whisk to combine.
Pour the strawberry gelatin into the holes in the cake. Then pour any remaining gelatin all over the cake. Refrigerate for at least 3 hours to let the gelatin set up and soak into the cake.
Remove the cake from the fridge and top with an even layer of Cool Whip. Add the sliced strawberries on top for garnish before slicing and serving.