During WW2 nerve and poison gas never ended up being used in the European theatre. Apart from a few rare and isolated incidents in case of the latter.
On the Axis side due to the Nazi leadership thinking the allies also had the knowledge and capability to manufacture and use them in greater quantities. While on the Allied side it was a prevailing sense of humanity and maintaining the moral high ground that resulted in usage not being authorised. (Even in the face of indiscriminate bombing and the V-attacks).
Assuming this however would have gone the other way, how bad could it have gotten? Especially on the western front.
Note that this assumes nerve gas being used against military and civilian targets before the British, Soviets, etc. (except for limited mustard gas reserves) would be able retaliate in kind. (Anthrax, area bombing of German cities, etc.)
How severe would the casualties have been? (Gassing Dunkirk or London, etc.)
And could it have forced Britain to the negotiating table?