Cover potatoes with salted water> by 2 inches in a 4- to 5-quart heavy pot and simmer until very tender, about 40 minutes.
Drain in a colander.
Mince garlic and mash to a paste with 1 teaspoon salt using side of a large heavy knife.
When potatoes are cool enough to handle when held in a kitchen towel, peel and force through ricer into cleaned saucepan.
Add butter, garlic paste, and milk and cook over moderately low heat, stirring vigorously with a wooden spoon, until fluffy and heated through, about 2 minutes.
Add cheese and pepper and cook, stirring vigorously with a heatproof rubber spatula or wooden spoon, until cheese is melted, smooth, and almost taffylike, about 10 minutes.
Preheat broiler. Butter baking dish. (If using 6 individual gratin dishes, butter and put on a baking sheet.)
Transfer potato mixture to baking dish, smoothing top.
Beat cream in a bowl with an electric mixer until it holds soft peaks, then beat in horseradish and remaining 1/4 teaspoon salt.
Spread horseradish cream over potato mixture and broil 4 to 5 inches from heat, rotating gratin if it isn't browning evenly, until golden, 1 to 2 minutes.
