Place potatoes, garlic and salt into a large saucepan, fill with enough water to cover then bring to a boil. Reduce heat to medium then simmer the potatoes until tender but not mushy, 8 to 10 minutes. With a slotted spoon, transfer about half the potatoes to an oblong baking dish.
Pour half of the cream over the potatoes, season with black pepper and nutmeg then sprinkle half of the Gruyere cheese onto the mixture. Top with the remaining potatoes; sprinkle again with black pepper and nutmeg. Pour on the remaining cream then sprinkle with remaining Gruyere cheese.
Bake in the preheated oven until the cheese topping is crisp and brown on top, about 1 hour.
You might want to put some foil on the lower shelf in the oven in case it spills over.