Your vehicle is composed of many intricate yet integrated systems. A problem in one may very well point towards a problem in another. Gas coming from your muffler is an issue probably initiated in the engine and not the exhaust.
Your muffler is one of the components of your exhaust system designed to lessen the noise crated by that exhaust. If you have fuel or gas coming out of the muffler, that means that the fuel is being introduced into the exhaust system itself where only gas emissions should be channeled.
Your engine is most likely an internal combustion engine. This indicates that fuel and air are sent into the engine cylinders where they are supposed to combust. In a properly functioning cylinder, the liquid fuel would then burn off. If there is a problem however, then the gas is not combusting fully and is being released through the exhaust system -- and eventually the muffler.
Experiencing this muffler problem may point toward a problem with the pistons in your engine cylinder. The piston goes through a number of strokes, the second being the "compression stroke." During this process, the piston forces the fuel into a smaller space, increasing the temperature and volatility of the fuel. If neither of these is happening, then the fuel will not burn off adequately. You should approach your chosen mechanic with these initial clues in mind.
