Preheat oven to 350ºF. Do not grease 9-inch by 13-inch pan.
Cool completely, at least 1 hour. Using a fork, pierce cake in rows about 1/2-inch apart. When the fork becomes sticky, dip tines in water to clean (typically at the end of each row).
Mix angel food cake mix according to package instructions (the one I used only required water). Pour into ungreased pan and bake until firm to the touch, about 30 minutes. If the center still jiggles, bake it 5 minutes longer.
Pour Jello into a medium bowl and whisk in boiling water. When Jello is dissolved, stir in cold water. Drizzle over the entire surface of cake. Cover and refrigerate at least 3 hours or overnight.
In a large bowl, or in a standing mixer fit with the whisk attachment, combine pudding and milk until well blended. Gently fold in whipped topping.
This high-rising, flavorful cake is perfect topped with strawberries and whipped cream. It's so tender and light, no one will suspect it's gluten-free!