Mark Ladner, the restaurant’s chef and widely considered to be one of the best pasta cooks in the nation, knows it is a remarkable number. Gluten, the protein in wheat that gives dough its elasticity, has been a key ingredient in his culinary success. But Mr. Ladner also knows that gluten-free dining remains a big and growing business, so he offers each of his pasta dishes, down to his 100-layer lasagna, in gluten-free form.