If the gas is leaking directly from the throat of the carburetor, that can be a sign of a needle and seat not properly sealing. Over time, contaminants will build up on those components and cause them to leak. A complete carburetor overhaul is usually the cure for this problem.
If gas is leaking from another part of the carburetor such as the float bowl, it could be a loose bowl nut or a bad gasket on that nut. The O-ring that seals the bowl to the carburetor could also be bad. Replace these parts and be careful to clean the grooves that hold the O-ring in place as buildup of dried fuel can cause that to leak as well.
If fuel is leaking from another part of the carburetor such as a jet screw, it is time to replace the carb with a new one. Make sure to order new gaskets when you replace the carburetor to ensure there will not be any air leaks. An air leak will cause the lawn mower engine to surge and run lean. This means it has more air than fuel.
Fuel leaks can be dangerous and lead to an engine fire. Take them seriously and if you need a lawn mower repair shop to assist with your problem, visit our Locations page for help.