I personally think that it was Putin that was responsible for the gas attack not Assad. right before the gas attack the United States was talking about disengaging the idea of regime change and the Russians need a destabilized Syria so they can keep there military post and port.
Certainly a valid probability.
What doesn't make sense to me though, why use gas at all, Assad or Putin? Was it for intimidation (of their enemies) purposes...or is it calculated to kill more per drop?