There has been no evidence presented that there was a gas attack before Donald Trump declared war on Syria the other day, but when there have been gas attacks in Syria they're invariably carried out by the US backed Al Qaeda militias they call the "moderate rebels." They did the attack that almost drew Obama into direct combat against Syria in 2013, that like this one was immediately blamed on Syria by the warmongers in Washington and their propaganda wing the US media. Actual evidence of rebel and Pakistani involvement that came out later was ignored by the mainstream media.
This piece by Scott Ritter enumerates the dynamics of why it happens -- it's when the US starts talking about getting out of Syria, as Trump Administration officials Nikki Hayley, the UN ambassador, and Rex Tillerson, the secretary of state, started doing March 30, a week before this latest attack is alleged to have occurred.
Recall that the media led the cheerleading for the war against Iraq and the one against Afghanistan before it by uncritically publishing every word of propaganda fed to them by the Bush Administration and its pro war Democrat allies.