According to the people he spoke to, Douma was a case of aphyxiation (from whatever cause) that was conflated by panic or malice into a gas scare.
This doesn't fit the deep state narrative. Given the suspicious timing of the Douma 'attack', I would bet on a cynically staged event.
Working theory: the White helmets deliberately gassed basement dwellers with carbon monoxide (by burning wood in a low oxygen environment) and then cried 'gas'.
Who knows the truth but it sure got Trump to reverse course from bringing our troops home.