The Douma gas attack has been investigated not only by the OPCW, but by organisations such as the UN, the Red Cross and pretty much all serious journalism from the BBC, NYTimes to The Times and Wall Street Journal. They all agree that the Syrian Governmant gassed its own people i Douma.
The consensus is overwhelming, the wiki page on the incident lists 119 sources. I obviously haven't checked all of them but most look credible to me.
“Expert Panel Finds Gaping Plot Holes In OPCW Report On Alleged Syrian Chemical Attack” by Caitlin Johnstone https://link.medium.com/7qYYVU2Z40
Back in 2012, when ISIS beheading videos where showing up on my Facebook newsfeed every week, I recall it was Syrian rebels who were brandishing chemical weapons - but the MSM never mentions it: https://youtu.be/TpIRRRuCEyg
I understand that journalism can't be all about debunking conspiracy theories, but Fisk does seem to have some clout.
https://www.independent.co.uk/voices/syria-chemical-attack-g...
Edit: This, attributed to the same author, is even clearer: https://www.counterpunch.org/2019/05/27/the-evidence-we-were...
----
"For in the last few days, there has emerged disturbing evidence that in its final report on the alleged use of chemical weapons by the Syrian regime in the city of Douma last year, the OPCW deliberately concealed from both the public and the press the existence of a dissenting 15-page assessment of two cylinders which had supposedly contained molecular chlorine – perhaps the most damning evidence against the Assad regime in the entire report.
The OPCW officially maintains that these canisters were probably dropped by an aircraft – probably a helicopter, presumably Syrian – over Douma on 7 April 2018. But the dissenting assessment, which the OPCW made no reference to in its published conclusions, finds there is a “higher probability that both cylinders were manually placed at those two locations rather than being delivered from aircraft”."