Is the boat still on the trailer? If so, remove the drain plug and open the hatch. If the hatch is electric, lift it by hand. Start with Castrol Super Clean and mix it with HOT WATER. Make it a STRONG solution and pour it in the bilge. If gas has gotten on other components like the engine, trim pump etc., wet them FIRST then spray with Super Clean and wash with soapy water. Repeat as necessary. I would do a final cleaning with or two with Simple Green and more hot water and soap as this will improve the odor.
Another alternative is to go to your local fire department and ask if you can buy or use some BIO-SOLVE. This is used to remove oil, gas and other vehicle fluids from roadways after an accident. It is Purple and yucky but it works GREAT!
Good luck. If you need any other help let me know.