Well, I've never torn one down, but almost all cylinder locks have a cam at the back end that you need to turn in order to move the latch, so you'll have to see if you can turn that somehow. I can't tell from the picture, is there anything left of the cylinder at the bottom of the hole? Do you still have the piece that got pried out?
Short of that, you'll have to get destructive. Weakest point is usually the hinge which you could cut through with a dremel or reciprocating multitool. It'd be a shame to do that since gas caps are scarce.