Locking gas cap

The dent puller one works well. It just pops the lock mechaism out, and then you can get the cap off easily. Or if you don'r have easy access to a dent puller, do as someone else said and jam a flat head screwdriver in the keyhole and get rough with it. You'll destroy the cap either way, but that's the plan. Don't pry behind the cap, spend all of your time on the lock itself. They come off easier than you'd think.