Hitler was personally against CW use but lets say when he learns the Allies have crossed into Germany in a fit of rage he authorises the use of CWs including nerve agents. I'm not sure who stepped first into Germany (WAllies or Soviets) but if it was the WAllies it happened in March of '45. The Allies had no idea Germany had a nerve agent program. Though adequate for phosgene and mustard, their CW protections are completely inadequate in the face of nerve agents as it only consisted of a gas mask. Many troops had long abandoned their gas masks by this point making matters worse. The Allies also have no clue about treating nerve agent poisonings. I anticipate the war in Europe lasts another six months with another half-million to a million WAllied casualties. The UK and US respond with their own chemical agents both tactically and strategically, and the UK goes Operation Vegetarian. The Allies rush in rubber chemical suits which hit the front in a month or two. The Soviets suffer just as badly. In the end, the war in Europe ends in nuclear bombings. Given Hitler didn't give up until the end I suspect it will take a lot more bombs to force them to capitulate than it did the Japanese.