Heat the oven to 375 degrees. In a 9-by-13-inch baking dish, stir together the cream, garlic, sage and nutmeg (if using) and season generously with salt and pepper.
Stir in the gnocchi and spread into an even layer (it’s OK if the gnocchi aren’t completely covered in cream). Sprinkle with the Gruyere and Parmesan. I like mixing some of the cheese, a quarter of each, into the gnocchi for the stretchy cheese factor.
Bake until the top is browned and crisp, 30 to 35 minutes. Let rest for a few minutes before serving.