Thats the engine breather hose.
Are you sure its gasoline and not water and oil?
Have you had a look at your carbs? It could be a stuck float in the carb, I would open the float bowls up, clean them and check the float valve shuts off.
If it is this it, maybe your carbs have been leaking fuel down the cylinders into the crankcase and hence then being pumped out of the breather hose.
Also remember to shut your petcock off as you may have fuel pouring into your engine from a stuck float.