This largely depended upon the type of gas used.

Against a blistering agent, such as mustard gas, which reacted with exposed skin, a gas mask was generally effective at protecting the eyes and face, but wouldn't help you elsewhere.

Against lethal agents like chlorine and phosgene, sometimes a mask was not even necessary. You could urinate on a rag and effectively counter a chlorine gas attack.

The gases used were also uniformly heavier than air, and thus sank to the low parts of the trench system. Thus, countering it could be as simple as crawling out of the trenches. This often exposed to you regular artillery fire, but as the gas could make staying in the trenches lethal, soldiers would take their chances. This actually had the beneficial side effect of often wiping out the vermin that inhabited the trenches and had no protection.

The major benefit of gas was psychological. One need only read a memoir (I rather enjoy Storm of Steel by Ernst Junger), to see the effect gas could have on troops who were unprepared.

There are quite literally hundreds of books that will provide you with the information you're looking for.

Keegan's The First World War and Meyer's A World Undone are rather solid introductory books to the subject.

For a first person perspective on the war in general and gas attacks specifically, I would suggest Storm of Steel. It's also an interesting counter-point to Remarque's All Quiet on the Western Front, in that Junger seems to have genuinely enjoyed his experience in the war.