In January 1917 he commanded the Infanterie-Regiment Kaiser Wilhelm Nr. 120 on the Western Front, fighting on the Somme. As commander of Infanterie-Brigade Nr. 18, he played a part in the fighting at Arras and Cambrai, and during the Spring Offensive. In 1918 he was promoted to Generalmajor and, on his request, retired from active duty in 1919. After the war, Gleich pursued a second career as a gifted and multitalented scientist. From an early age, as an officer cadet, he had demonstrated interest and aptitude in mathematical and astronomical studies. In the post-war era he wrote around 40 scientific papers on physics and astronomy, contributing significantly to debates on relativity. In 1930 he published a comprehensive overview of his work in the book "Einsteins Relitivitätstheorien und physikalische Wirklichkeit" ("Einstein’s Theory of Relativity and Physical Reality"). Gleich died on 7 April 1938 in Ludwigsburg, following a long and painful illness.