What's the fastest, easiest, cleanest way to kill millions of people? You'll find that "gassing" them comes in around number 80 kajillion down the list.

Just look at the gas chambers that were used to carry out capital punishments here in the US. They're giant pressure vessels with doors that look like they belong on a submarine. Exhaust fans to evacuate the gas, fume dilution fans on the roof to safely dispose of the spent gases. As you said, the safety of the prison workers was a primary focus. These facilities have nothing in common with a gang shower room with wooden barn doors.