Preheat oven to 400 degrees F (200 degrees C). Grease a 9x12 inch baking pan.
Melt butter over low heat. Stir in flour until smooth. Gradually stir in milk and cheese until cheese is melted. Season with garlic powder, salt, and pepper. Remove from heat. Arrange the leeks in a single layer in the prepared pan and cover with the cheese sauce.
Bake for 30 minutes in the preheated oven, until leeks are tender and sauce is bubbly.