On May 1, 1933 he joined the NSDAP. In his role as constructor at Großfuß during the 1930s Gruner is credited with the invention of the well-known MG 42 general-purpose machine gun, the development of which was ordered by the Waffenamt after shortcomings of the MG 34 had been recognized. Named like its predecessor after the year of entry into active service, the MG 42 first supplemented and later replaced the MG 34 in the last years of World War II.
