From 1902, Gerngross was commander of the 1st Siberian Infantry Brigade, which was subsequently active in combat during the Russo-Japanese War of 1904–1905. He was promoted to lieutenant general in 1904. During the Battle of Te-li-Ssu, his division came under a strong artillery attack from the Imperial Japanese Army, and suffered heavy losses. Although the promised reinforcements never arrived, he personally led two regiments in a successful counterattack on Japanese positions. He was wounded in the battle, and was subsequently awarded the Gold Sword for Bravery. He was also at the Battle of Liaoyang.