I love history. I read many books on World War 2 years ago and moved on to other areas. I thought that the gas chambers were a very settled question. I usually stay away from fringe sources, but I recently watched: "Adolf Hitler: The Greatest Story Never Told" on youtube. https://www.youtube.com/watch?v=VhGfCTA_6wA There was an obvious agenda here so I took it all with a grain of salt. What concerned me was the discussion of the gas chambers and how there was lack of proof for this and many of the proofs may have been fabrications by the Soviets. I know this is a delicate subject. I would love an unbiased take on this.
Have a look at the judgment from Irving v Lipstadt from 7.15 to 7.76 which summarises the gas chambers at Auschwitz
The case concerned a libel claim from Irving against Lipstadt for particular claims that he was a Holocaust denier and anti-Semite. The sections cover wartime intelligence about the gas chambers, evidence gathered from prisoners and camp officials, evidence from several post-war trials (including the Nuremberg IMT and Eichmann trial in 1961), documentary, photographic and physical evidence from Auschwitz about the gas chambers in refuting Irving's claims that the gas chambers were not used to kill Jews, or were not used to kill Jews on a large scale.
You can also read this: https://www.jewishvirtuallibrary.org/jsource/Holocaust/denial1.html
Oh, and watch the 9 hour documentary Shoah. Or watch the brief film Night and Fog. It's probably on YouTube. It contains footage the Nazis took of the concentration camps.