Cook the gnocchi according to package directions. Drain well and pour into a greased 2 quart casserole dish.
Preheat the oven to 375 degrees.
Make the cheese sauce: melt butter over medium heat. Whisk in the flour, and stir for 30 seconds, until mixed with the butter. Whisk on and off for three minutes, and then add the half & half, stirring briefly to combine. Allow to thicken for three minutes, and then whisk in the dijon mustard, garlic powder, nutmeg, and salt and pepper. Turn off heat and stir in the cheddar, gruyer, and parmesan cheese.
Pour the cheese sauce evenly over the gnocchi. Sprinkle the breadcrumbs over the top. If you wish, sprinkle some additional parmesan cheese over the breadcrumbs.
Cook uncovered in the preheated oven for 35 minutes, until bubbly. Let sit for five minutes before serving.