The gas cap door won't open,clip popped out.

Sounds like it needs to be liberated.

There is no other release. Take it to a mechanic or the dealer. Or else just force the door open to reach the cap. Is it just a door cover or the fuel cap itself?

FIRST: try using the gas release lever from inside the car with the driver side door closed. My 2006 Jetta gas release lever would only work if the driver door were shut and I pulled the release from the inside car interior. Turns out it was a severed door wire (NOT THE RELEASE CABLE). Thus when the door was open, it stretched the damaged wire and so none of the door controls worked unless the door was closed. ie windows, trunk release, and gas release. It was a $600 repair at the dealership, parts and labor! :(

Gas cover has a pressure clip that fits inside on the gas door latch. When you pull on the cable in the cabin a little arm withdraws and the pressure clip causes the gas door to spring open. If the clip is missing, the arm withdraws but there is no spring to pop the gas door open. Replace the pressure clip.

Pushing the door with your hand isn't going to help, but after digging through a lot of info online, I succeeded in doing this. You have to peel back the lining of the trunk on the right side. Once you get a chance to look in there, there's a somewhat transparent white box inside the trunk near the gas cap. Attached to that is a small metal arm that connects to the inside of the gas cap release.

Tug on that piece of metal, and you'll pop open the gas cap.

Most cars have an emergency fuel filler door release somwhere in the trunk. Look for some kind of pull cord close to the area of the outside fill door.

Try removing the plastic lever handle to see if it will let you push the lever further towards the floor resulting in more tension on the (hopefully not broken) cable.

Take a plastic knife like you would get at Wendy's (or anything you can think of that would work better) and wiggle it in the center of the right side of the fuel door. You're trying to depress a trigger and pop it open. Forcing can bend the inner latch but it can be bent back. After you get it open you'll need to do some adjusting or diagnose if the cable is broke. Even though it's the same lever, it's two cables.

pop the trunk, same side as gas door pull the interior carpet down or back. Look for a wire with a loop on the end, pull it. "The door should open

The cable back to the release has probably stretched to the point it won't release (hey it's 18 years old!). Get a pair of vise grip pliers, grab on firmly to the cable behind the release you are pulling up on, and pull, it should open.

Try this: Have someone pull the release cable. Using a small tool (credit card, plastic knife, anything); see if you can manually "pry" (for lack of a better term) the filler door open. If it DOES open, the fuel door "pop" spring is either broken or compressed to the point that it will not "pop" the door open. The spring is a flat piece of metal, bent into sorta of a "U" shape, and is located at the hinge side. Try this, and get back to me if you need more assistance. Hope this helps.
Toyota Ed