Serving with distinction during the Russo-Turkish War (1877–1878) he received a field commission as brevet captain, which was formally confirmed in 1879.

During the Battle of Te-li-Ssu, his division came under a strong artillery attack from the Imperial Japanese Army, and suffered heavy losses.

[1] At the Battle of Shaho, Gerngross commanded the reserve forces, which he used to recapture positions from the Japanese south of the Sha River before being ordered to withdraw.

During the Battle of Mukden, Gerngross (who had just been reinstated as commander) and General Konstantin Tserpitsky, despite a lack of reinforcements, led a counterattack, which temporarily halted the advance of the Japanese Third Army.

Made a member of the Military Council on 20 November 1913, he then commanded the 26th Army Corps in World War I, from 15 August 1914 to 28 December 1916.