You need a new fuel door lock assembly. GM part # 96534241. Typically around $10 online. If you try to shut the fuel door too quickly or forcefully the little stem that holds the door shut will snap off. These are easily replaced from outside the vehicle. Two small ears on assembly allow you to turn assembly quarter turn to right and assembly will pull out exposing assembly and about two inches of cable. Snap assembly off white collar on end of cable housing and pull cable end out of assembly. Make sure you hold onto cable and don't let it fall inside or you'll be removing interior body panels to retrieve it. Depress lock stem on new assembly and insert cable end. Snap assembly back on white collar of cable housing. Make sure assembly is secure on cable and replace assembly and turn quarter turn to left. Latch should work now. Good luck.