Preheat the oven to 325°F. Grease a 2-quart baking dish with butter. Rub the cut side of the garlic all over the bottom and sides of the dish. Slice the potatoes thinly.

Arrange a layer of potato slices on the bottom of the dish, slightly overlapping them. Pour a little cream over the potatoes and scatter some Gruyère cheese on top. Season with salt and pepper. Continue layering the potatoes, cream, and cheese until all ingredients are used, finishing with the cheese.

Bake the gratin in the preheated oven for 1 hour, or until the potatoes are soft and the top is golden brown. Let it sit for 10 minutes before serving. Sprinkle with parsley.