Mr. Death: The Rise and Fall of Fred A. Leuchter, Jr. (1999) documentary

Documentary about Fred Leuchter, an engineer who became an expert on execution devices and was later hired by revisionist historian Ernst Zundel to prove that there were no gas chambers at Auschwitz. Leuchter published a controversial report confirming Zundel's position, which ultimately ruined his own career. Most of the footage is of Leuchter, puttering around execution facilities or chipping away at the walls of Auschwitz, but Morris also interviews various historians, associates, and neighbors.

You can also look:
The Truth Behind The Gates Of Auschwitz - David Cole in Auschwitz 1992. Documentary
https://rumble.com/v6tt5mj-the-truth-behind-the-gates-of-auschwitz-david-cole-in-auschwitz-1992.-docum.html
Auschwitz-Birkenau No Gas Chambers by Father James Mawdsley https://rumble.com/v6ty61d-auschwitz-birkenau-no-gas-chambers-by-father-james-mawdsley.html