Place rack in middle of oven and preheat to 350°F. Place potatoes in a large wide heavy pot. Add 1 1/2 tsp. salt and enough cold water to cover by 2″. Bring to a boil over high heat, then reduce heat to medium-low and simmer until potatoes break apart when pierced with the tip of a knife, 3540 minutes. Meanwhile, cook garlic, cream, and butter in a medium saucepan over medium heat, whisking occasionally, until butter is melted. Whisk in nutmeg, pepper, and 1 Tbsp. plus 1 tsp. salt. Remove from heat. Drain potatoes and immediately pass through potato ricer into a large bowl. Stir in cream mixture until smooth. Return half of potato mixture to wide pot; reserve remaining potato mixture in bowl. Add Gruyere and mozzarella and cook over low heat, stirring constantly with a wooden spoon, until cheese is melted and mixture can be stretched with the spoon (like warm mozzarella), about 10 minutes. Transfer to gratin dish; smooth top. Working in batches if needed, spoon reserved potato mixture in bowl into pastry bag fitted with star tip. Pipe very tight rows of 1/2×1/2″ rosettes on potato-cheese mixture in gratin dish. Bake until top is golden brown, about 45 minutes. Let stand 10 minutes before serving.