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My favorite source for all WW1 information is the YouTube channel The Great War, which follows the events of the war week by week exactly 100 years later.
They have an episode for the week gas was first used in a large scale, the week from the second battle of Ypres, and a special on just poison gas. These will be my sources for my answer.
The first large scale gas attack took place in January of 1915 by the Germans against the Russians, but because it was deployed in the frozen eastern front, it wasn't very successful due to the gas failing to vaporize. This was tear gas, not chlorine, mind you. Chlorine was first used in the second battle of Ypres, as you've been reading. The French saw the advancing yellow cloud, and figuring it was a smoke screen meant to obscure advancing troops, sent their troops to the area to defend. Because it was poison, this is what opened the giant hole in the lines that you mention in your question. The second episode linked goes into this at about 2:30. It was combination of reasons:
Cl
gas had been used, so they weren't sure what to expect.