There is a Spiegel Online video from earlier today with the assessment of Christoph Sydow, who is kind of their leading reporter on Syria. He is convinced this has been an attack by regime forces. I thought his account is quite interesting, as he talks about sources and provides some details, so I have summarized and translated it. All of the following is solely Sydow's portrayal & reasoning, if someone has contradicting information, please share it.
On the credibility of the reports of a gas attack:
According to Sydow, several different sources from Douma confirm fatalities and sickness from poison gas, including doctors, white helmets, reporters and eye-witnesses, and all of these reports match.
On the possible course of events:
According to eye-witnesses, there was a grenade attack from a helicopter on saturday between 19:30 and 20:00, and the victims' symptoms have started to occur in the following minutes and hours. At least 46 persons have died.
Sydow says all the previous claims can be considered safe information. He further says if the gas attack has indeed been committed by helicopter, it could only have been regime forces, as they are the only ones to operate helicopters over Douma. A ground-to-ground missile attack would leave rebel forces as possible perpetrators as well, but there are no indications for this to have happened.
On possible motives and benefits for Assad:
For years now, it has been part of the regime's warfare to spread fear and terror among the population. The message for the Syrians is: you can only lead a relatively safe life inside the dictatorship if you don't oppose it. All who resist are subjected to collective punishment, examples being the starvation of towns, previous gas attacks, and the destruction of whole villages.
As affairs stand, the attack was beneficial for Assad in the short term, as shortly after the attack the Islamists of Jaysh al-Islam have agreed to withdraw to Northern Syria. Assad has therefore cleared the area of enemy forces, and seized the town without having to engage in urban combat.
On the chances of repercussions for Assad:
Trump and Macron have condemned the attack and described it as (again) the crossing of a red line. Sydow says he nevertheless can't see a serious threat for Assad's regime emerging from this. It is possible that the US and France decide to strike Syrian military facilities in the coming days. But that wouldn't change anything about the fundamental situation in Syria, which is Assad continuing his rule with the support of Iran and Russia.
Harsher measures would lead to a serious political clash with Moscow, which Sydow doesn't consider to be in Trump's interest. So for him, symbolic military action is the most likely consequence, which would neither influence the balance of power in Syria, nor prevent possible future gas attacks.
