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.
Whilst I think the German advances in the field would have been an ugly shock, as others have flagged the allied counter measures would have had some impact. I think the losses on either side would rise significantly, but the German civilian population might be the worst impacted.
The big problem Germany faces is that the Allies have far more delivery options available. The Allies might be delivering an inferior gas, but they kept the chemical weapons response ready to go, and had more artillery and aircraft available. The weaker parts of the German army would have probably been a bit lacking in protective kit, and with the fighting taking place in and around German cities the impact would have been horrendous.
Others here probably have a better recollection. I think if I remember correctly an army wants mainly persistent agents when on the defence, and dispersing agents on the attack. Was the German program well suited to fighting on their own soil?
Hitler might face another attempt by the military on his life if he attempted to use them, the downsides would be pretty obvious to the German military.