Preheat oven to 375 degrees F.
Bring 2 cups of the heavy cream to a simmer in a medium saucepan. Add the potatoes, salt and pepper and cook for 3 minutes.
Rub the cut side of the garlic clove on the bottom and sides of an 8-inch casserole or baking dish. Add the potatoes and cream and top with the remaining cream and Gruyere cheese. Sprinkle chives over top, if using. Bake on a baking sheet until potatoes are tender and cheese is golden brown and bubbly, about 30 minutes. Remove from oven and let rest 10 minutes before serving.