Similar gluten-free dishes, like pasta made with rice and corn starches and chewy focaccia, are woven into the menus at all the restaurants owned by Del Posto’s proprietors, Mario Batali, Joe Bastianich and Lidia Bastianich, Mr. Bastianich’s mother. “It really has become a thing, and I don’t think it’s going to go away anytime soon,” Mr. Ladner said.