Position a rack in the center of the oven and heat the oven to 350°F. Butter a 9x13-inch (or similar) baking dish.
In a 5-quart pot, bring the milk, cream, nutmeg, and garlic to a boil over medium-high heat. Reduce the heat to medium low and simmer 5 minutes, stirring occasionally, to infuse.
Meanwhile, melt the butter and pour into a medium bowl. Add the breadcrumbs and Gruyère and toss.
Stir the potatoes, 1-1/2 Tbs. salt, and 1/2 tsp. pepper into the milk mixture, raise the heat to medium high, and return just to a boil. Carefully transfer the potato mixture to the prepared dish and spread out evenly. Scatter the breadcrumb mixture over the top and bake until the potatoes are tender when pierced with a fork, and the breadcrumbs are golden-brown, about 45 minutes.
Let rest 15 minutes before serving.