First, let me ask something to clear things up a bit, you say you don't use the attached display, instead you use an external monitor. Does the attached display itself not work? Or are you just using the external by choice?
I ask because if the attached display works (and just using the external monitor by choice) then you should be able to try using just the attached display to see if it works or not. This would tell you if the problem was the GPU or the external monitor/cable.
If the attached display was the reason why you use the external monitor, then I would try a different cable and different monitor to test it and see if it is one of those or the GPU. There is no way to tell without trying. Or you can take it to a local tech and they can do all the trials and work, but that would cost.