Preheat oven to 400 degrees.
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 Gruyere 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 Gruyere cheese.
Cover with aluminum foil lightly sprayed with cooking spray.
Bake at 400 degrees for 20 minutes.
Remove from oven; remove and discard foil.
Preheat broiler and broil gratin for 5 minutes or until cheese is brown and bubbly.