Had this issue on my 7.5 and I resolved it very easily in about 2 minutes. Dealer wanted to replace the gas door, hard pass from me as it would have to be painted and all that. So while I was working at a VW dealer I talked to one of the master techs and he told me exactly what to do.
When you open the gas door, the top of the tab that goes into the locking mechanism has a little bit of plastic that needs to be trimmed off with a knife or box cutter. I am going to try and link a picture from Golfmk7 forums that shows what area needs to be trimmed just slightly. I recommend doing a small amount first and see how the door functions.
