There were gas chambers But those weren't designed to gas humans en masse A 1950 US gas chamber for 1 guy https://www.mv.helsinki.fi/home/tuschano/cp/wyoming.gif https://media-cdn.tripadvisor.com/media/photo-s/01/04/69/86/have-a-seat-in-the-gas.jpg You want to gas 6 million people? That's not the easiest way to proceed, and that's far from cheap, and it's rather dangerous