I'm reading up about the second battle of Ypres (April-May 1915) which started with a chlorine gas attack north of the Ypres-salient.
The attack caused a gap of several miles (3 to 4) wide to be opened. Their only obstacle to capturing Ypres (& overrunning the entire salient) was the G.H.Q line. The G.H.Q - line although ideally situated on top of the Pilckem Ridge wasn't adequately staffed at the time and could thus be taken if attacked by a large enough force.
But I read that the Germans didn't have enough reserves to deploy & to capture Ypres, let alone exploit the breakthrough to its fullest. Nor were they apparently fully aware of the potential effects/success of the gas attack. They wasted precious time observing the effects of the gas before attacking with infantry. In the end the opportunity to move in and capture Ypres was lost.
The gas itself was already used in a combat situation at Bolimow (though it partially failed) and was probably (couldn't find records on this) tested in labs. So they must have known the potential of the weapon.
So this leaves me with the following question: Why didn't the German High Command foresee the possible effects of the gas? And why play out your trump card (the gas) in the first place, if you don't have the manpower to fully exploit a possible breakthrough?