Gustav Franz Wagner (born July 18, 1911 in Vienna - died 1980) was an SS officer and deputy commandant of the German Sobibór death camp in Poland, where more than 200,000 were gassed during Operation Reinhard.
From the evidence I have see, there has never been any concrete, verifiable evidence of any 'mass gassing' of Jews during WWII...the only real expert who ever inspected Aishwitz and other sites, an American who designed the gas chamber used to execute prisoners in California, USA, stated that, from his observation, that there was absoluely no physical evidence for any gas chambers...the American expert who exposed this was Fred Leuchter.
Here is a link you can start with:
ihr.org/leaflets/gaschambers.shtml...but I warn you, if you really want to know the truth about this, you will have to spend some time reading.