Vice grip ang katapat nyan. Tapos bumili ka ng bagong gas cap, get a receipt and show it to the gas station manager for a refund, (in cash or kind). Hopefully hindi na damage ang threads ng gas tank mo mismo.

Next time when filling up, especially when you're driving an "older" car, take time to go out and open the gas tank yourself. Most of the gas boys right now are just too careless (i.e. nasanay sa quick release gas caps ng mga bagong kotse).

My gas cap is the lockable type, wherein it needs to be opened by the ignition key. So I really need to turn off the engine, go out and open it. They are more careful when they know you are watching them.