Cook the gnocchi according to package instructions. Drain and set aside.
While the gnocchi cooks, get started on the cheese sauce: In a small bowl, stir together the cornstarch and the broth until evenly combined and set aside.
In a large skillet, over medium heat, heat the butter over medium heat. Add in the garlic and saute, stirring constantly, about 1-2 minutes. Stir in the cornstarch mixture, and whisk until combined and a roux has formed. Slowly stir in the milk a little bit at a time, 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 too are melted and the cheese sauce is smooth. Season the sauce with salt and pepper, to taste, if desired, and remove from heat.
Transfer the cooked gnocchi to the cheese sauce stirring until they're completely coated. Serve immediately, with a sprinkle of finely chopped, fresh basil and/or parsley and a sprinkle of shredded Parmesan, if desired.