Preheat oven to 350 degrees F and grease a 9x13 inch baking dish.
Boil the elbow macaroni in salted water until 1 minute shy of al dente, then drain and set aside.
In a large saucepan, melt the butter over medium heat. Whisk in the flour and cook for 1 minute to create a roux.
Gradually whisk in the warm milk, stirring constantly until the mixture thickens and lightly simmers.
Remove the saucepan from the heat and stir in the salt, black pepper, garlic powder, grated cheddar, and gruyere until the cheese is completely melted and the sauce is smooth.
Fold the cooked macaroni into the cheese sauce until well coated.
Transfer the mixture to the prepared baking dish and sprinkle the panko breadcrumbs evenly over the top.
Bake for 20 to 25 minutes until the edges are bubbly and the breadcrumb topping is golden brown.