Hitler absolutely despised gas. He had been blinded during a mustard gas attack at the end of WWI, and the experience led him to conclude that poison gas was a completely evil weapon. He never used it, even as Germany was falling apart and his own generals were urging him to deploy it he refused. It may well have been the only line he was unwilling to cross.
The one time we can be grateful to Hitler for anything honestly; as I can't even begin to imagine the horror that would have happened if a less restrained leader had gained control and unleashed the gas.