Preheat oven to 350°F (175°C). Grease a 9x13-inch pan.
Prepare the vanilla cake mix with eggs, oil, and water as per box instructions. Pour into the pan and bake for 25–30 minutes (until a toothpick comes out clean). Let cool for 15 minutes.
Poke holes all over the warm cake using a skewer.
Dissolve strawberry gelatin in 1 cup boiling water, then stir in ½ cup cold water. Slowly pour the mixture over the cake, letting it soak into the holes.
Refrigerate the cake for at least 3 hours (or overnight) to set.
Spread whipped topping evenly over the chilled cake. Garnish with sliced strawberries. Slice and serve cold.