A garage that smells like gasoline can be due to a few potential sources of gas. The most obvious source is your car; if you store your car in your garage, you may want to examine the gas tank for leaks. After that, home landscaping tools such as trimmers and lawn mowers have tanks that can leak as well as gas cans with vents that may be open or damaged.
Your gas tank is usually located underneath your car's trunk. Examine the tank and the pipe that runs up to the gas cap by running your hand over the area and feeling for dampness. The smell of gas should be stronger under your car if the tank is the source of the leak. Do not attempt to plug the leak yourself; take your car to a dealership or mechanic for repair.
Lawn Mowers and Trimmers
A lawnmower or trimmer contains gasoline not only in its gas tank but also in its carburetor and the gas lines running from the tank. If your trimmer is lying on its side or hung on the wall in such a way that it is not level, it may be leaking. Wipe down the trimmer or the lawn mower and, if possible, leave it outside overnight. Return to your garage the following day, and determine if the smell has dissipated. If it has, you likely have a leak in your trimmer or mower. Return it to an authorized dealer for repair.
Many gas containers -- the red containers you use to refill your trimmer or mower -- have vents to enable easy pouring. These are small nipples on the opposite side of the container from the spout. They need to be open to allow easy pouring but will leak gas fumes if allowed to remain open. Check your containers and ensure these are closed to eliminate the odor.
Gas Spills and Odor Permanence
Note that removing the source of the gasoline smell will not remove the smell immediately. Your garage floor, especially if it is unsealed cement, will hold a good amount of gasoline for quite some time. You may continue to smell the fuel, though to a lesser extent, for days or weeks after the source has been removed. If the smell is reaching inside your house, use a solvent to clean your garage floor.