Preheat oven to 400°.
Place potatoes in a large Dutch oven. Cover with water; bring to a boil. Reduce heat, and simmer 15 minutes or until tender; drain. Return potatoes to pan. Add 3/4 cup fontina cheese, 1/2 cup Gruyère cheese, butter, salt, and freshly ground black pepper to pan; mash with a fork or potato masher until well combined. Add warm milk to pan; continue mashing potato mixture until desired consistency. Spoon into a 13 x 9-inch baking dish coated with cooking spray; sprinkle evenly with remaining 1/4 cup fontina cheese and remaining 1/4 cup Gruyère cheese. Cover with aluminum foil lightly sprayed with cooking spray. Bake at 400° for 20 minutes. Remove from oven; remove and discard foil.
Broil gratin 5 minutes or until cheese is brown and bubbly.