Melt 3 tablespoons butter in a medium saucepan, add leeks, cover and cook over low heat for 5 minutes, until tender. Uncover, stir in flour and cook 1 minute. Add milk in a thin stream, whisking constantly, and simmer 5 minutes, stirring occasionally. Remove from heat and add cheeses and mustard, stirring until cheese melts; season to taste with salt, pepper and nutmeg.
Meanwhile, rub a buttered oval gratin dish with garlic clove. Layer half of potatoes in bottom of dish, season with salt and pepper and top with half of kielbasa. Add half of leek sauce, spreading it evenly over sausage and potatoes. Repeat with remaining potatoes, sausage and sauce, generously seasoning potatoes. Top with buttered bread crumbs and dot with remaining tablespoon butter. Bake gratin until top is browned and bubbly, about 40 minutes.