Did the folder " PMDG 777-300ER in the \AppData\Roaming\virtuali\Airplanes exist before you put the GSX.cfg file there or did you create the folder?
For me there was no folder.. I manually created it but it does not work, cargo is loaded on top of the airplane.
Do I need to run the FSDT updater first perhaps to let GSX know there is a new aircraft?
Its not there from beginning, no.
Be careful about capitals etc and also that you are really at the right place. And inside the Airplanes folder you ned to have exactly one folder. THe one of the airplane. And inside that the cfg file.
I say that because sometimes zips contain another subfolder around the real one.
That said, i wouldnt bother with it so far. At least it doesnt work for me.
It loads to correct places but if i pushback like that the nose is getting stuck.
BUT, that was with 3.0.4. Not sure if that also happens with 3.0.5-