Probably the carburetor is leaking. Use a flashlight and your hand to locate leak.And which of the over 5000 variation of a Briggs engine is leaking gas. Without model number info we have no idea what your engine looks like or where to start guessing the origin of the gas.
I just had a CR950 the other day that would appear to leak gas from the fuel bowl when the tiller that the engine was attached to was tipped forward. The tank actually had a 1 inch long crack about 4 inches from the carb.Probably the carburetor is leaking. Use a flashlight and your hand to locate leak.