EXTREMELY IMPORTANT: When rekeying a gas cap DO NOT CLOSE THE GAS CAP until you are 100% positive that it works correctly with your key. If you close the gas cap before being sure of this then you risk not being able to open the gas cap again. To verify correct operation of the gas cap the tab shown below should retract when you turn the key counterclockwise.

In order to make the lock core work with a particular key insert your "new" key and then file down (I use a Dremel grinding wheel) anything that sticks out except for the very last tumbler as that is the one that holds the lock in place.

Example:

Then reinsert the lock core into the gas cap with the side of the core with the jagged side of the key pointing forward: