On the other hand who would benefit from a gas attack? Obviously, the "rebels" (read islamists). They have been losing ground for the past two years and if the US pulled out (which was planned) and a peace settlement is reached then they would lose.
Now consider that the exact same sequence of events happened almost exactly the same time last year -- the US announced that "regime change is no longer a priority" and a few days later we saw reports of a gas attack. See http://www.moonofalabama.org/2018/04/timelines-of-gas-attack...
For the game theory see: http://econlog.econlib.org/archives/2018/04/from_game_theor....