I'm sure this has been done before, but here's a quickie 5min thing to do if your gas cap key goes missing like mine did.
There's a metal ring around the inside of the rubber gasket which you must remove first.
This gains access to 3 screws, which when removed, allows the locking mechanism to detach from the tank mount.
Once removed, remove the rubber seal between them, and the lock cylinder slides right out
Then remove the pins with pliers (they all come out one way), slide it back in, reassemble, and you are good to go!
(keep in mind, you will have to drill it first or pick it if its locked on your tank)