1. The Syrian opposition has much to gain including the increased likelihood that Western powers might get more involved in the fight if they believe Assad did it. Big gain for them. 
2. Undoubtedly the bombing was by either Russia or Syria. But the explanation that the bombs hit a munitions factory producing chemical weapons is certainly more plausible than Assad deliberately launching a chemical attack at this point. He had nothing to gain from it, and much to lose. See point one.