If you look at the photos above, there is a bevel to the "pin" that allows it to get pushed in as the door closes and then pops out again to keep the door closed. When you pull the cable the pin retracts and the door pops open due to the spring that is supposed to be in the hinge but looks to be missing from your door - but it is hard for me to be certain. The spring is in the photos above too so you can verify if yours is missing. As
@azngarage stated, it may just be that your "pin" is retracted due to a stuck cable from the gas door release lever. If your lever is loose or still in the open position that may be a clue. It seems unlikely that the pin would break or wear down. If you look in the nut, you should see the pin there. To access that area, you should be able to remove the door where the jack is stored in the rear quarter panel and access that area without having to take off the actual interior trim.
