Jeremy Paxman and Robert Harris did a book on this, and both it and its' bibliography are extremely useful; there are several inconvenient facts, some of which the men of 1940 were well aware of, some not.
Much of gas attacks' prompt tactical effect came simply from the fact that it was a cloud of opaque vapour; mixing smoke shell with gas shell to extend the term of a gas attack beyond the filter saturation point and oxygen tank life of the defenders had already been highly successful at Vimy; smoke alone could have much of the same effect.
Most of the damage actually done by inhaled gas was not prompt, but expressed itself in later life, when the gassed had retired from the army and were approaching middle age, when the respiratory damage done by exposure really started to make a difference to their lives.
The short term effects of gas could be mostly produced by smoke alone, which had very little of the long term effects, and was also much cheaper. Certainly much cheaper than gases capable of promptly and severely injuring large numbers of people; things like the later elaborations, nitrogen mustard and so on, the arsenic based D series agents, which were highly lethal, were also expensive and available only in small amounts.
The allies were dimly aware of this, but not of the extent of German superiority, larger stockpiles of more lethal agents- the allies did not suspect the existence of the organophosphate G series at all. They knew that the Germans were well prepared, just not how well.