Look, you have the testimony regarding how gassings were done. And you have among other studies, Richard Green's export report on how these things were possible given the physical chemistry.
But I want you to engage in a thought exercise, please.
Here's what a prison gas chamber looks like:
http://media-cdn.tripadvisor.com/media/photo-s/01/04/69/86/have-a-seat-in-the-gas.jpg
Presumably, you know what the Birkenau gas chambers looked like.
Green's report is here:
http://www.holocaust-history.org/irving-david/rudolf/affweb.pdf
Begin reading on page 25.
I will presume, however, that you don't have the attention span, so:
In a prison gas chamber, the cyanide level reaches 7,500 ppm. That is about 75 times a lethal dose required. In the Auschwitz gas chambers, it is likely (though not certain) the level never reached that high. Instead, efficacy of killing was achieved, in part, by the fact that the chambers were so crowded.
To wit: I believe you are Japanese. Ever been to Tokyo? If so, ever ridden the subway there? During rush hour? Now image a room twice as crowded, and you get some idea of what the gas chambers would have been like. Now drop cyanide pellets on the floor and have people start panicking.
Getting the picture yet?