what was in it for Assad ? He's winning the war, why use gas and get the western countries all pissed?
Well, because he can. The real answer to your question is against civilian populations, chemical weapons are instruments of terror, a demonstration that resistance will bring horrific consequences. Assad doesn't care about the outrage from West. He's protected by Russia and he knows they are not about to invade Syria and get into a confrontation with Russia.
The Syrian army had already won. They had defeated Jaish al Islam (another AQ affiliate) and freeing the civilians of Douma. Why the hell would Assad gas the very same people they've been fighting to free for years?
And Russia at the time of the gassing was negotiating a cease fire. The motive is just not there. And why the hell should we ever believe any terror group affiliated with AQ and financed by the Saudis?