1/4 teaspoon finely-ground black pepper, or more to taste
(optional toppings: finely-chopped fresh parsley or basil, extra Parmesan cheese)
Cook the gnocchi in a large stockpot of boiling water, according to package instructions. Drain and set aside.
Meanwhile, as you are waiting for the water to come to a boil and for the gnocchi to cook, make your cheese sauce. Begin by whisking the cornstarch and the vegetable/chicken stock together in a small bowl until the cornstarch is completely dissolved. Set aside. (Be sure that the stock is room temperature or cooler, otherwise the cornstarch will not dissolve.)
Heat butter or olive oil in a (separate) large saucepan or saute pan over medium heat. Add garlic and saute for 1-2 minutes, stirring occasionally, until fragrant. Stir in the cornstarch mixture, and whisk until combined. Add in the milk, and whisk until combined. Continue cooking, whisking constantly, until the mixture reaches a low boil. Then remove from heat, and stir in the cheeses until they are melted and smooth. Season the sauce with salt and pepper, to taste.
Combine the cooked gnocchi and cheese sauce in the large stockpot, and toss together until the gnocchi are evenly coated. Serve immediately, topped with optional toppings if desired.