There are just as many, and equally credible, accounts that say the "rebel" forces were the ones who did the gassing, if gassing was really done. This is something fairly easy to fake and there hasn't been any solid proof, is anyone reminded of Iraq here?
We do know that they have access to Sarin and we know that they are perfectly willing to kill unlimited numbers of civilians, including children. They have done this many, many times and continue every day.
Even if we believe that the Syrian government is culpable that doesn't warrant the US taking action to help the "rebels" who are mostly foreign mercenaries drawn from Al Qaeda, Hezbollah, Taliban and other Islamic extremist. These groups are the enemies of the US, sworn to bring down the West and everyone who isn't Islamic.
Obama's end goal, or more correctly the profiteer puppet masters who's strings he dances on, is to draw Iran into war with the US & NATO. Another unwinnable war to fill their coffers with unlimited $trillions.
These vipers don't care who wins or loses, who gets killed or how many lives are destroyed so long as they get rich.
There is absolutely no reason for the US to get involved and 92% of Americans are opposed to any involvement at all.