On August 17, 1939, Gerlach was drafted into the Wehrmacht as a reservist where he was promoted to unteroffizier and appointed leader of telephone-construction crews. From February until April he was posted to Halle an der Saale for officer cadet training. Following that, he was with the 1st Signals Battalion in Königsberg until August. For the rest of the year, Gerlach served in the 228th Signals Battalion in Westphalia, being promoted to Leutnant on September 1st, 1940. From December 1940 to April 1941 he was a platoon leader of the same division and deployed to France. In April 1941, Gerlach and his platoon took part in the Balkan campaign in Yugoslavia and in June of the same year he transferred to the 16th Infantry Division, which took part in the attack on the Soviet Union on June 22nd, 1941. In July 1942, he was promoted to Oberleutnant and placed on the staff of the XXXXVIII Panzer Corps where he participated in several battles, including the Battle of Kiev, the Battle of Moscow, the double envelopment battles of Vyazma and Bryansk, and Case Blue. From the end of July, the corps belonged to the 6th Army which was attacking toward Stalingrad. On October 24th, 1942, he was transferred to the 14th Panzer Division where he worked for the divisional intelligence officer. His responsibilities included assessing the enemy situation, deployment of subordinate units of defense, letter censorship of subordinate units of the secret field police and attached units of the Propaganda Company. During heavy fighting in the city centre, the 14th Panzer Division lost almost all of their tanks and stood in the section between the Bread Factory and the Volga River. Gerlach was severely wounded in the head and was taken prisoner of war by the end of January 1943.