My car's got gas!

I've got a 97 Sunfire that smells like gas. I especially notice it when it's parked in the garage. No visible signs of leaking and the car runs just fine. It's got to be coming from the front of the car because while driving I can get rid of the smell inside by shutting off the fan. Turn the heater (or fresh air) back on and wham, there's the gas smell. Any clues to what it might be?

Thanks