Gas smell, decreased mileage, check engine light

My girlfriend's '99 Passat Wagon has a gas smell after the car has been running for a while, and is not getting the same mileage off of a tank that she used to. She said it's getting under 300 miles on a tank as opposed to about 350 before she noticed the problem. Last night the check engine light went on. Do you have any suggestions as to what we might look at? Any ideas?