Using a Screwdriver When There's No Key
If you don't have a key, you can use a flathead screwdriver to remove the locked gas cap. First you will pry off the chrome key hole cover with the screwdriver. Then you can pry the lock mechanism out and remove the gas cap. It shouldn't take too long or too much strength to use a screwdriver to open your locked gas cap. You can also use a screwdriver to remove the lock completely, or you can wedge the tool into the lock's grooves and start turning it to unlock it. When you use a screwdriver, you will probably destroy the locked gas cap, and you will need to purchase a new one. A tire iron, which you may have more readily available if this happens on a road trip, can also be used instead of a flathead screwdriver.
Or...................
Another method you can try, which will destroy the gas cap like the screwdriver method does, is to drill a 1/8-inch hole through the outer part of the gas cap and insert a sharp, pointed drywall screw into the lock. When you feel resistance, you should have locked the two halves together, so you can then screw off the entire gas cap.
http://www.ehow.co.uk/how_5525214_remove-broken-locking-gas-cap.html