Why is gas coming out the muffler of your push mower when you try to start it?

Sounds like you have a weak spark or a fouled plug that is not burning the fuel completely and the fuel is escaping into the muffler via the exhaust valve. This could be dangerous (heat + fuel = danger). Check the spark plug, the spark plug wire, etc for a good spark - the contacts on the magneto could be dirty or rusty too. Another reason could be a stuck fuel float in the carburetor - you will have to remove the carburetor to check.