This falls under what is called Holocaust Denial, which its adherents inappropriately call Holocaust revisionism in an attempt to validate their arguments. Be assured, there is no historical validity to what they believe.

This is how I responded to a question regarding the validity of historians statements on the Holocaust in this post.

deniers use difference in scope to create initial confusions into which they insert more confusion. Finally they end with "so, you can see that someone doesn't want you to know the truth."

The Holocaust ISN'T disputed. It IS fact. What I mean by that is no well studied person doubts or denies that 1) the Nazis perpetrated the planned persecution and then planned extermination of ROUGHLY 6 million Jews and thousands of Roma (commonly called Gypsies) through neglect, overwork, starvation, summary executions, mass shootings, and the use of gas chambers; and 2) the Nazis persecuted and killed MILLIONS of others, which resulted in millions of deaths through neglect, overwork, starvation, summary executions, and mass shootings. The only researched individual who would "dispute" this is someone with an agenda.

The agenda for such dispute is NOT academic although most deniers wrap their denial in psuedo-academic language (hence the term "revisionism"). Their denial is due to supremacist ideology with a belief in a Jewish conspiracy.

So, what does this mean for this priest? For whatever reason, he is a Holocaust denier. If someone is a Holocaust denier, do not engage him/her. It is pretty much useless because he/she has ulterior motives for such beliefs in almost all cases. However, I will go point by point over why this bishop is spouting absolute nonsense.

He first says that 6 million Jews weren't killed in gas chambers. This is true. However, no Holocaust historian would ever make the claim that 6 million were gassed. There have been some that said such large numbers were killed in gas chambers, but they were not historians of the Holocaust. For example, Auschwitz commandant Rudolf Höss initially stated that 2 million Jews were killed by gassing at Auschwitz. This has been studied and found to have been an overstatement that he himself retracted. 6 million is the total estimated number of Jews killed by gassing, shooting, starvation, disease, etc. during the Holocaust.

He next uses the Leuchter report. He gives great credibility to the author of the report by stating that he was an engineer who worked heavily in designing gas chambers for executions. However, he failed to note how the report has been discredited and that Leuchter was shown to have lied about his credentials.

He then speaks about the difficulty of working with cyanide. However, he misstates certain details in order to argue his point. First, "they are wearing some clothes." No, they weren't. In almost all cases, the victims were nude. They chambers also had fans which sucked out the gas after use. "The doors must be airtight and the doors there are not airtight." He is speaking of doors other than the metal gas chamber doors used to seal in the victims.

In other words, he is absolutely ignoring all evidence and basing everything on, in this case, a single, discredited testimony.

If you have more questions about Holocaust denial, feel free to ask. As I said before, however, do not engage a denier.

Lipstadt, Deborah. Denying the Holocaust New York: Plume, 1993.

Sorry if my first answer may have missed part of your question. You also seem to be asking how they used gas chambers. If so, this answer is for you...

Prelude

Prior to the gassing of the Jews, the Nazis had gone through what many historians consider to be a series of "solutions" to the Jewish question. Whether planned from the outset by upper-level Nazis or originating with those "on the ground," the Jews of Germany had first been subject to legal persecutions and physical assaults on themselves and their property. It seems that a purpose of these persecutions was to cause the Jews to emigrate to other countries. The Nazis allowed emigration of Jews and the only limitations they put upon it was the amount of money the Jews could take.

Things changed when the Germans came to control Poland. Upon conquering western Poland, Germany gained a huge Jewish population. The various leaders of the territories established did not want the Jews in their territory and this led to a large amount of infighting. Ultimately, most of the Jews of Poland were collected in ghettos. The Jews of the western European states conquered were all treated in a variety of ways, but we will focus on Poland's Jews first. There were a few various ideas for what to do with the Jews that included using them as slave labor based in work ghettos, starving them in the ghettos, sending the Jews to Madagascar (a death sentence since it was estimated that only 10,000 people could live on the island), or resettling the Jews in the east at the successful conclusion of the war with the USSR.

Operation Barbarossa (the invasion of the Soviet Union) seems to have led to a rapid radicalization of German policy. During the invasion, special units known as Einsatzgruppen were responsible for rounding up Jews and shooting them in open air massacres, such as at Babi Yar. However, Himmler discovered that the men were often damaged by these tasks. Historians disagree on the exact time, but sometime between late 1941 and early 1942, plans coalesced for the destruction of the Jews in Europe through death camps.

Death Camps

To help get understanding of the difference between death/extermination camps and concentration/work camps, you can see my answers here and here

In short, the Operation Reinhardt camps that were the first extermination camps began by using carbon monoxide and then cyanide to poison victims. These victims were sent to the camps for the sole purpose of extermination. Once the Jews of Poland were exterminated, for the most part, the camps were dismantled. Auschwitz-Birkenau II was the site where most of the Jews of the other parts of Europe were killed (as well as the remaining Polish Jews). Jews sent to Operation Reinhardt camps were exterminated immediately. Jews sent to Auschwitz might go through a selection in which the young or strong might be put into the work camp area while the rest were exterminated. They also might simply all be exterminated. It depended on such variables as the needs of the factories in the work camps, the availability of the crematoria, or the whims of a Nazi official.

The Process

Now we get down to the real answer to the question. How did they kill the Jews in the chambers?

First, the victims were brought to Birkenau and offloaded from the train cars or marched there from other parts of Auschwitz (such as in the example of the "family camp"). Once there, the victims might have known what awaited them, as in the case of Polish Jews from nearby areas or the Family Camp, or they may have been told that they were in a delousing/cleansing area and that before deportation or work they needed to shower. In the former case, the SS would beat the victims into submission if they failed to undress and enter the gas chamber. In the latter, an SS officer would tell the victims that they should undress in preparation for cleansing. An elaborate ruse was worked out in which there were hooks with numbers. The officer would tell the victims to remember their numbers so that they could get their belongings afterward. Once undressed, the victims were moved into the gas chamber, complete with false shower heads to continue the ruse. The metal, rubber sealed door was closed behind them.

Above the gas chamber were SS guards with canisters of zyklon-B (an insecticide), gloves, and gas masks. They would open the canisters and drop the pellets down into the columns of the chamber. Meanwhile sonderkommando, Jewish forced laborers who worked in the crematoria, began sorting the belongings left behind in the undressing area. An SS man would check to see if all the victims were dead (roughly 20-30 minutes after gassing began) before the vacuum fans were turned on. This released the zyklon-B from the chamber. After the gas was gone, the sonderkommando had the unpleasant task of taking the bodies to be incinerated. First, however, the women's hair was shaved and the gold teeth were removed by sonderkommando. The process for removing the bodies from the chamber was difficult. Since the deaths were via gassing, the victims were often piled up on top of one another in an attempt to escape the gas. The sonderkommando therefore had to untangle them. The floors were watered so that the bodies would slide more easily. Bodies were put onto an elevator to raise them to the furnace level. The gas chamber was cleaned of all bodily fluids and often whitewashed to keep the appearance of a shower chamber.

The bodies of the victims, now bereft of all value to the Nazis, were then put on a metal platform on rails. More than one body was placed in a furnace at a time. Filip Mueller, a sonderkommando, claimed that an experiment was done in one of the crematoria to determine the most efficient allotment of corpses. It found that a healthy body combined with an emaciated body and a child would have enough fat (from the healthy body) to keep the fire burning with the minimum amount of fuel. The platform was rolled into the crematorium where it dropped the corpses. A metal "rake" was then used to make sure the bodies stayed in the furnace. After burning, the ashes were cleaned out and new bodies put in. The ashes were buried in the area around the crematoria or were put into nearby waterways.

This was the established process as of early-1943 when new crematoria were built to handle the increasing work load. Prior to that, victims were gassed in smaller chambers and were buried. These bodies began to swell and stink in the summer of 1942 and were cremated outside.

Feel free to ask any further questions you might have.

Sources

Muller, Filip. Eyewitness Auschwitz: Three Years in the Gas Chambers. Chicago: Ivan R. Dee, 1999.

Nyiszli, Miklós. *Auschwitz: A Doctor's Eyewitness Account." London: Panther Books, 1969.

Friedlander, Saul. Nazi Germany and the Jews, 1939-1945: The Years of Extermination. New York: Harper Perennial, 2007.