Preheat oven to 375 degrees F (190 degrees C).
Rub the bottom and sides of a 3-quart baking dish with Garlic (1 clove), then mince garlic to be used in cream sauce. Grease dish with Unsalted Butter (as needed).
Transfer Yukon Gold Potato (4 pound) and garlic to a large pot with Heavy Cream (2 cup), Whole Milk (2 cup), Fresh Thyme Leaves (1 tablespoon), Coarse Salt (1 1/2 teaspoon), Freshly Ground Black Pepper (1/2 teaspoon), and Whole Nutmeg (1/8 teaspoon). Bring to a boil over high heat. Reduce heat to medium and simmer for 1 minute.
Let cool slightly, about 10 minutes.
Use a slotted spoon, to transfer one-third of the potato mixture into the prepared dish. Sprinkle with one-third of Gruyère Cheese (6 ounce). Repeat layering twice more.
After layering the potatoes and cheese, drizzle the liquid over the top, letting it seep down into the dish. Loosely cover the dish with parchment and then cover with foil that has been sprayed with nonstick cooking spray.
Bake until potatoes are fork-tender, 35 to 40 minutes.
Remove foil and parchment; turn oven to broil. Broil until potatoes are bubbly and browned in places, 3 to 5 minutes.
Let cool 30 minutes before serving. This 30 minute resting time is important for the most perfect texture and flavor of the dish.