The Answer:

Concrete is porous, meaning that it has lots of little holes in it for the gasoline to soak into. Many professional garages paint their floors to make it resistant to staining and soaking by fuels, etc. In your case, you may have soaked up most of the gasoline by wiping it up, but you are smelling the evaporation of the volatile chemicals in the gasoline which has soaked into the floor.

You should ventilate the area as well as possible; leave the garage door and any windows open, especially if your garage is attached to or underneath your house! The high volatility of gasoline and its components not only makes it explosive, but it can also be a health hazard. Once the fumes have dissipated for a day or so, you should clean the garage floor with a patio or driveway cleaner, which is available from your local hardware store.

—The Editors