There’s a lot of sigint that our spooks are privy to. Russian, Syrian and Iranian disinformation aside, it’s 100% certain that the Syrians did this. And the entire point of the gas attack is probably aimed at getting Sunni Arabs to leave the country.
Look at how much trouble the Palestinians have caused the Israelis, and there are fewer of them than Jews in Israel. Whereas Sunni Arabs outnumber Alawites 7 to 1 in Syria. Sunni Arabs have been in periodic armed revolt against Alawite rule for 50 years, ever since the first in a string of Alawite rulers, Salah Jadid https://en.wikipedia.org/wiki/Salah_Jadid, took power in 1966.
The current civil war is simply the closest the Sunni Arabs have come to taking power. The problem for the Alawites is that they have not come close to inflicting a 7 to 1 kill ratio* on the Sunni Arab population. That is why - just to maintain the demographic balance prior to the revolt, they need to force a large number of Sunni Arabs out. The gas attack may or may not have helped, but it couldn’t have hurt in that effort.
* My guesstimate is that at the very best, they’ve managed a 1.5 to 1 kill ratio. The pre-war Alawite population was about 2.5m. They’ve lost perhaps 100K Alawite mostly combatant dead. At 4% of the total, that’s a huge number. Japan capitulated after the A-bombs in 1945. By that point, they had lost 3.5% of their population from both combat and Allied strategic bombing. Japan’s eventual defeat wasn’t in doubt - the Allies had done nothing but advance against Japan in the Pacific after the Battle of Midway in June 1942.
We used A-bombs on Japan after losing 400K dead (300K fighting Germany), 0.3% of the US population at the time, despite the certainty of victory because we were just tired of losing men. Assad, after losing perhaps 4% of the Alawite population to Sunni Arab rebels, is way past tired of losing men. Even if the rebels nominally hand over some of their weapons to the regime (while burying a good bunch of it in caches around the country), Alawites can’t stay in power in a country with an Iraqi-style low level insurgency crippling the economy. He needs large numbers of Sunni Arabs to leave permanently.
Sunni Arabs were so close to power they could taste it. That is why large numbers must be evicted forever, if this Alawite victory is to be anything but a Pyrrhic one. And gassing them is probably the best way to encourage this population movement, much as Menachem Begin’s massacre at Deir Yassin successfully convinced a large number of Palestinian Arabs to depart Israel ahead of the combined Arab invasion of 1948, thereby establishing Jewish demographic superiority in Israel after the Arab armies were defeated at huge cost in Jewish lives, relative to their numbers at the time.
Your thinking is too clever by half. Like your Israeli example showed, you don’t need chemical weapons to get people to leave. Gun down all the adult men, rape the women and children. People will get the message and leave. Starvation is an even more effective tool to precipitate large scale population transfers. Immediate hunger and thirst will force undesirable Sunnis out of Syria faster than a random once a year chemical weapon attack.
The biggest problem with the American strategy in Syria is fundamentally because the Americans demand that they be the “goodies” and other people be “baddies” and thus the mountain of lies peddled by the American luggenpresse regarding America’s role in Syria.
The most beneficent reading of America’s intention in Syria is that the CIA/Pentagon/State Department honestly thought a foreign Jihadi backed Sunni radical movement could affect a relatively quick and painless regime change to a more Pro-American government and that simple bureaucratic inertia kept with this strategy for years past its sell by date even when it became obvious it wasn’t going to work. This is plausible because the American establishment is precisely that combination of delusionally stupid and cravenly careerist that this is a realistic explanation.
The alternative hypothesis is that the CIA/Pentagon/State Department knew exactly what it was doing in funding Jihadists and forming an alliance of opportunity with Al Qaeda and really has no expectation of actually overthrowing Assad. The entire purpose of the War is only tangentially related to Syria and it is instead primarily a method to bleed Iran and to a lesser degree Russia of men and material at relatively modest financial cost to itself. Afghanistan redux. The hundreds of thousands of Syrian dead are simply irrelevant roadkill in this strategy.
The rather bold faced lies coming from American state media could be reconciled by either explanation. The first theory is that the lies are to shield their gross incompetence. The second theory is that the lies are to shield their Machiavellian scheming which soft hearted American voters wanting to be the good guys would never publically approve of.