During WWI, the Germans weren't shy of using it on the battlefield and during WWII, it would have probably helped the Germans in the Russian campaign seeing as their equipment was lacking and could have been a turning point in the battle of Stalingrad. It can't have been for ethical reasons or the Geneva convention since the Nazis didn't care much for those.
Hitler didn't want the Wehrmacht using poison gas (possibly due to the effects he had seen whilst serving in the trenches - I think he was temporarily blinded at one point).
http://rense.com/general83/gas.htm
http://www.historynet.com/why-no-poison-gas.htm - also, no,effective counter measures.