No evidence that Assad is responsible for the gas attack, what's more there is no credible reason why he should do so at this moment, but every reason why many would wish to discredit him might.
The 'regime change' in Iraq was a disaster, ditto in Libya, why do you think this will be any different? Who will replace him ? everyone wants someone else for religious and political reasons.
Please go and study the history of the Arab world in general and Syria in particular before you start coming up with naive answers.
I can offer you an extensive reading list if you want.