Its popularity as one of the greatest American musicals has too often eclipsed its stature as the Great American Opera, which is what George Gershwin was aiming to write. Nothing argues his case more persuasively than serious treatment from America’s premier opera company. Pretested to raves in London, James Robinson’s new production brings the ambitious score back to the Met stage for the first time since December 1990, with Eric Owens and Angel Blue heading a powerful new cast. —M.G.