That's my opinion based on a cross section of information. I have heard the Russians, British, Americans, various groups in Syria regime and otherwise and my judgement based on the logic of whether it would be in his interests to do so, as well as some independent observers like the ex UK ambassador.
My considered opinion is no way Assad was involved, and the British and American governments are lying when they said he was and attempted to use it as a pretext for escalation. Just like wmd in Iraq, we were lying there too.
You haven't given your opinion.