Weather PODs are fine by me. My only problem was that I confused Ypres and Neuve Chapelle! What am I like?
IOTL not a few Germans were rendered hors de combat by blue-on-blue as a result of the gas. Some commanders might still try to use it ITTL, with similar effect. It'll certainly hold up the German attack, since the Entente forces won't retreat/die en masse and thus the situation won't require a substantial redeployment. I wonder if it might be one of those PODs that has a greater impact down the line rather than immediately. The Brits and French don't realise the potentially catastrophic use of gas, and so end up on the back foot at a later date. Then again, this is post NC, when Haig has not only realised gas' utility, but overestimated it. So maybe not.
I suppose it might lead to a cancellation of the various German schemes of attack, and so lower casualties all round.