Gross was tried and convicted of a single charge of manslaughter in the 1950s, but the verdict was overturned by a higher court, which ordered a retrial. The retrial never happened.
Authorities reopened the case after fresh evidence, including declassified papers, was released two years ago from the archives of the former East German secret police.
Gross admits to referring children to a euthanasia board but denies that he himself had ever killed anyone.