That seems to be an official counter-narrative, this question as to what Assad would have to gain from gassing his own people, given the potential for a response. Well, he gassed his own people before, and there was NO response. Mr. Barack “Red Line” Obama did nothing.
Therefore, it is entirely believable that Assad would gas his own people once again, figuring the U.S. would not respond.
It may be Saddam Hussein you are remembering, he gassed the Kurds. Same Baath Party as Assad at least.
I didn’t find a reference to Assad using gas before when I looked for one last night but he certainly killed civilians in other ways. They just didn’t cross the line into a method now regarded as a war crime.