Gasoline stains on concrete, whether old or new, can be treated much like oily spills on concrete. Cleaning requires several steps, due to the absorbent nature of concrete (especially if it is not sealed concrete). Although tough, concrete can be damaged from harsh cleaning methods--however, there are gentle methods to remove gasoline stains from concrete surfaces.
Ventilate the area, if the stained concrete is inside your garage or home. Pour clay-based cat litter over the gasoline stains, unless the stains are very dry. The cat litter will absorb some of the gasoline--leave it on the stains overnight.
Sweep up the cat litter and place it in garbage bags, away from heat sources and open flames, for disposal later (contact your trash company before disposing of this in your regular household garbage--you may need to take it to a hazardous waste facility).
Combine powdered laundry detergent, in a bucket, with a little bit of water to form a thick paste. Make enough of this detergent paste to cover all the gasoline stains. Choose a detergent that claims to remove grease stains for best results.
Pour the detergent paste over the stains. Leave it on overnight.
Pour warm water over the detergent and stain and scrub the area with a nylon scrub brush or nylon scrubbing sponge. Rinse the area with water. (If this is your driveway, use your garden hose. Otherwise, rinse with water from a bucket.) If any stain remains, re-apply a paste of powdered detergent and again leave it overnight, before scrubbing it once more the following morning.