You could ask the ones who went through the gas chambers at Auschwitz and other so-called "death camps" - only, there were no gas chambers.
The only "proof" of gas chambers is testimony from people who were there. But just "being there" doesn't mean there were gas chambers. Most people actually believed there were gas chambers - not from seeing them, but from the propaganda that the BBC had been broadcasting over the radio which these people heard before they were captured.
Once they got to Auschwitz the men and women/children were separated, both going to segregated areas of the camp, many never to be heard from again. And because they never heard from their family/friends again they automatically surmised that they had been taken to the gas chambers.
What most likely happened was that they had caught typhus or one or more of other diseases that were of epidemic proportions. But under the circumstances, combined with the propaganda that they were indoctrinated with, they honestly believed that there were gas chambers.
Other than this, there were no signs of any other gas chambers other than the one that the Soviets staged to implicate the Germans in crimes against humanity - even the curator of the Auschwitz museum admitted that the gas chamber was rebuilt and that the holes in the roof were not original. This lie has been accepted as a historical fact because it was never questioned by the Western Allies.
Prisoners being held in Germany, both liberated and who had died showed the same emaciated state as those of Auschwitz - their health issues were obviously of the same origin. But it was proven that no gas chambers existed anywhere in Germany.
And with the only "proof" of a gas chamber being the one staged by the Soviets, it's highly unlikely that it could be considered verifiable as proof of anything. What you have left is no proof of gas chambers except the testimony from people who supposedly "had no reason to lie" - except, after being locked up against their will and being made to work for their keep, they had *every* reason to lie.