It has been suggested that Assad used the gas because of a recent Trumpian pronouncement Tillerson  that Syria was not an immediate concern of the US....(thus Assad need not worry about US intervention if Assad further violated basic human rights of his people.)

I find that typically the exact opposite of what Trump says is true....

CNN: a few days ago

Indicating a possible shift in US policy on the war in Syria from the days of the Obama administration, Secretary of State Rex Tillerson said on a trip to Turkey that the "longer-term status of President Assad will be decided by the Syrian people."
And in New York, US Ambassador to the UN Nikki Haley was even stronger about the Trump administration's decision not to push for Assad's departure. "Our priority is no longer to sit and focus on getting Assad out," Haley told wire reporters Thursday, according to AFP.