Preheat the oven to 400 degrees F.
In a small saucepan, combine the cream and garlic powder, and season with salt and pepper. Heat until just shy of simmering.
In a 3- or 4-quart gratin or casserole dish, layer half of the sliced potatoes, heated cream and Gruyere. Repeat, then sprinkle the top with the Parmesan, pressing to even.
Cover with foil and bake for 1 hour. Remove the foil and continue baking until golden brown on top, 10 minutes more.