You don't say where the gas is leaking out of but I would assume from out of the carburater overflow tube. The problem is either a sticking or improperly adjusted float or a malfunctioning float needle. The float moves the float needle either to close or open the valve to allow gas to enter or stops the flow depending on the specific height of the gas in the chamber. Any excess will drain out the overflow tube and onto the ground. You should fix this asap before the bike catches fire!
