2004 Pontiac Grand Am Smells like gas but no leaking

When the car is running and sitting still you can smell gas inside and out. We have checked for leaks and there are none. Gas mileage is still the same and hasnt changed. It has progressivly gotten worse over the past 3 months. Friends said the O-ring may be cracked? How much is that kind of repair dont they have to take apart the engine? What else could it be? So confused and tired of the horrible smell. Thank you, please help.