1. Preheat oven to 350°. Stir together first 5 ingredients in a medium bowl. Let stand 5 minutes.
2. Arrange a single layer of potatoes in a buttered 13- x 9-inch baking dish. Pour 1/4 cup cream mixture over potatoes; sprinkle with 1/4 cup Gruyère cheese. Repeat layers 7 more times. Top with Parmesan cheese.
3. Bake, covered with aluminum foil, at 350° for 45 minutes. Uncover and bake 15 to 20 more minutes or until golden brown and potatoes are tender. Cool on a wire rack 10 minutes.