Chemical warfare certainly took the "enemy" - in this case allied soldiers - by surprise. However, some gases - mainly lachrymators in the form of tear gas - were used in tiny, tiny numbers by the French and Germans in 1914. They were used in such tiny quantities that the French shells were never detected, but the German shells were, somewhat. However, it was never deemed an issue so no PPE or drills were brought into service. Why would they? It wasn't a threat. As we know this all changed.

The 1899 Hague treaty, however, did cover chemical weapons. IV, 2 of the Treaty, Declaration concerning the Prohibition of the Use of Projectiles with the Sole Object to Spread Asphyxiating Poisonous Gases, prevented the usage of "the use of projectiles the object of which is the diffusion of asphyxiating or deleterious gases.".

This leads to the question, why did they not issue protection otherwise? Well, this was because gas had never been used in great quantities, and protective equipment wasn't exactly widespread. That's why we see how basic the first gas clothing in WW1 was - there was simply a lack of knowledge about gas as a weapon and the protection that could be given to the individual.

So far, a TLDR is thus: Gas wasn't well known as a weapon, and so protective gear was not made. The Hague Convention of 1899 said no to chemical weapons, and many powers signed it. It was assumed no chemical weapons would be used, and they were too unknown to be protected against.

As for the poison effects of chlorine gas, this part is true and hard to answer. We can say however that the effects of gas were unknown for that mass battlefield scale.