Gruner was born on June 7, 1904, in the village Terpitzsch, then belonging to the municipality
Zschadraß, which since January 1, 2011 is part of the town Colditz on the banks of the
Zwickauer Mulde near its confluence with the
Freiberger Mulde to form the
Mulde 3.5 km north of the town. His father taught in a
Volksschule and he attended the
Realgymnasium in Döbeln. when he became a technical designer for the
Metall- und Lackwarenfabrik Johannes Großfuß (
Metal and lacquerware factory Johannes Großfuß) in
Döbeln. 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. In 1943 Gruner was appointed as lecturer in the field of
non-cutting shaping of sheet metal at the
Braunschweig University of Technology, an appointment to the
RWTH Aachen University in late 1944 was hindered by the advance of the Western Allies. In May 1945 he witnessed the arrival of the
Red Army in Döbeln. == Post-war decades ==