Born in Leipzig in 1928, Gerlach was arrested at school in 1944 for starting an anti-fascist youth group. After World War Two he joined the Liberal Democratic Party (LDP). This was one of the small political parties that the communist government allowed to exist. He studied law at the Walter Ulbricht German Academy of State Sciences and Law. During the 1950's he became General Secretary of the LDP and the mayor of Leipzig. From 1967 to 1990 he was on the Council of State, and its vice-chairman. He became chairman of the LDP in 1967 until it merged with two other parties in 1990, to take part in the first free elections in East Germany.