It can be a few different things.
First thing that I'd check is the fuel lines underneath the car. Are there any drips?
The next thing I'd check is under the hood. Any leaks there? Just follow your nose to find it.
If you can't find any leaks or drips, you might have a hole in the top of your gas tank. They tend to rust through on top. The holes on top of a gas tank usually aren't big holes like you would see on a fender. They're frequently little holes, like the ones on a salt shaker. You'll notice the smell getting stronger just after filling up the tank.
You probably shouldn't run the car until you fix the leak.
