As you raise the gas door on a C4 there are two springs towards the front that tend to break, this set of springs is the answer.
Unfortunately if the springs in your 1984-1996 Fuel Filler Door break, you can't buy a new assembly,,,,period. There's none available, so you must buy a set of these springs, remove gas door assembly from car, do some work that is a little complex but doable.
After installing, door will stay in up position when opened like it used to.