To the best of my knowledge, no proof has been presented.  I fault Trump for acting without meaningful investigation.  Analysis of motives to use CW is not just speculation.  Military gain and loss is pretty much obvious.  The military gain to using CW was minimal, the damage to international relations gigantic.  The US has no on-site investigators.  The UN has refused to condemn the Syrian government owing to lack of evidence and clarity.  Thanks for the conversation, but note that you said nothing that motivates the use of CW by the Syrian government.  You simply state that you believe the authorities.  Okay fine, some people are like that.  I would hate to have you on a jury where taking the word of authority would mean an automatic guilty verdict.