I have gone into the history of the usage of gas before here

Heinrich Himmler, after watching members of the SS shoot Jews or other "undesirables" during the campaign in the east, remarked that he felt sickened by what he had witnessed. He felt that killing Jews in this way was too strenuous on soldiers and that an easier, more dehumanized way of killing would be preferred. As a result, the method of killing Jews via gas vans was introduced. A precursor to gas chambers, gas vans were personnel transport vans that had their exhausts modified to feed the engine exhaust (carbon monoxide) into the back of the vans, which would be filled with Jews, communists or other demographics targeted for extermination by the nazis. With this method, a few Germans could go for a drive and kill enemies of the reich on the way. In this way, German soldiers did not even have to look at victims while they killed them.

Sources: Ian Kershaw, "Hitler: A Biography" New York: W.W. Norton & co. 2010.