Someone with more expertise can explain this better than I can, but it's not quite that simple.

What actually happened, though, is that a chemical weapons arms race developed, as each side developed more sophisticated types of countermeasures and the other side was forced to innovate newer and nastier type of chemical warfare agents to bypass the countermeasures:

In 1914, there is small-scale use of non-lethal irritants like ethyl bromoacetate.

In 1915, the Germans experiment with genuinely lethal gasses, and settle on chlorine for large-scale use in what became the Second Battle of Ypres. Chlorine was toxic, but only when inhaled in large quantities. Also, chlorine gas is easily detected, being strongly odorous and a dark green color. As chlorine is water-soluble, a simple but expedient gas-mask can be made of any peice of folded cloth that is soaked with water (or urine) and held against the nose and mouth.

Thus by November of 1915, both the French and the Germans were switching to phosgene (Carbonyl dichloride), which is considerably more lethal than chlorine, and also much harder to detect since it is colorless and has only a faint "cut grass" smell that easily blends into the background smells. Phosgene was mixed with chlorine in battle field use, and the early gas masks used sodium phenoxide, which was only partially effective in reacting with (and negating) the carbonyl chloride in the phogene mixtures.

In response, by January of 1915 the British had to developed a new generation of gas masks that used hexamethylenetetramine (called hexamine as the time -- modern organic chemical nomenclature wouldn't be invented until the 1920s).

Thus by 1917 the Germans had to find ever-more toxic chemical warfare agents. The most widely used agent in the later phases of the war was "mustard gas" (aka sulfur musturd, aka Dichlorodiethyl sulfide, formally Bis(2-chloroethyl) sulfide). Unlike chlorine and phosgene, which are choking agents that attack the lungs only, mustard gas is vessicant, a blister agent. This means that mustard gas will cause chemical burns and blistering on any exposed skin, including both dermis (hands, arms, face, torso, etc) and mucous membranes (eyes, lungs, mouth, sinuses, etc).

It was mustard gas that would kill over 100,00 people in the Great War, and about which Wilifred Owen famously wrote in his poem *Dulce Et Decorum Est.

More insidiously, both phosgene and mustard gas often have delayed-onset health effects (most commonly respritory distress) at sub-lethal doses -- these injuries don't become evident until a day or two after initial exposure. Mustard gas can remain active, potent, and dangerous for as long as several weeks in standing water and soil. It's heavier than air, so will pool and condense in small puddles in low areas, where it can remain dangerous to everyone for days after it's initial use by one side or another.

Finally, wearing a gasmask limits your vision and also tends to make breathing more work since you have to pull air across a series of filtering membranes. So even after effective gas mask were distributed, an attack with poison gas could still be part of a larger combined-arms attack to disrupt a defensive position.

So you see, it's not quite so simple as "gas mask makes you invulnaerable to gas attacks".