The Mauthausen gas chamber was ready in the spring of 1943, and until the may 5th 1945 liberation it has gased almost 3500 people. To avoid panic, it was designed as a shower, an ordinary bathroom. The door that lead to the gas chamber had a hole that allowed SS forces to watch the prisoners die, live.
“While most inmates were killed by shooting, hanging, beating, starvation, and disease, Mauthausen did have a gas chamber capable of killing about 120 people at a time. The gas chamber was usually used when transports of prisoners arrived. Special demonstration mass killings were organized for the benefit of visiting Nazi dignitaries, such as Heinrich Himmler, Ernst Kaltenbrunner, and Baldur von Schirach, who were able to observe the killings through a small viewing window in the entrance door.” [SOURCE]