1999 Mitsubishi Galant broken door for gas cap

Recently the door to the gas tank -- the door that gives access to the gas cap -- on my car broke. It still opens and closes, but it does not stay closed. If there's wind, for instance, it's blown open. I have two questions. First, is there a special name for this door? Second, what might I do and where might I go to get this fixed?

Thank you.