Preheat oven to 375˚F. Spray shallow 9"x9" baking dish with nonstick spray. Arrange cooked gnocchi in a single layer at the bottom of the dish.
In a saucepan over medium heat, melt butter. Stir in garlic and cook until lightly golden, about 1 minute. Whisk in flour until mixture thickens, then add milk and sage. Continuing to whisk, cook sauce until thickened, about 2 minutes.
Add shredded cheeses to the sauce. Stir until melted, then season with salt and pepper. Pour sauce over layer of gnocchi. Top evenly with Parmesan and breadcrumbs. Bake until bubbly, about 25 minutes. Allow gnocchi to rest for 5 minutes before serving.