I had the same problem today (elantra 2005).
Step 1. Push the gas door lightly in, while pulling the release lever, so the closing mechanism has less friction. My arms are too short to reach fuel door and lever at the same time, so I had to pull the lever with a plastic bag. Now let the door swing open.
Step 2. The plastic closing toggle holds on to a thin piece of sheet metal bent as a hook. Bend the hook more backwards, so the plastic closing mechanism has less grip of the fuel door.
It worked for me.