Marking the largest private gift in the hospital’s nearly 100-year history, South Shore Health System today received a $10 million donation that will help it fight the public health crises surrounding substance abuse and mental health. The donation will allow South Shore Health System, the parent organization that operates South Shore Hospital, to launch the Grayken Center for Treatment, named after benefactors John Grayken, a Cohasset native, and his wife, Eilene Davidson Grayken. The couple in 2017 made a $25 million donation to Boston Medical Center for the same purpose.