At a Boston synagogue Tuesday night, hundreds of people gathered to press health care executives on their efforts to control health care costs. A year into a new Massachusetts law aiming to rein in costs, it's still unclear whether the state will be successful. But the Greater Boston Interfaith Organization, or GBIO, a self-appointed health care cost watchdog group, met Tuesday at what amounted to something of a "people's hearing" and pledged to hold leaders accountable. The topic: health care spending. The place: a basement meeting room at Temple Israel in Boston.