I go to the FMC, open them and nothing happens! They never get opened.
Do you mean you clicked on the FMC to open the doors, and they don't open ? If this is the case, that's not obviously a problem of GSX, which in fact is working correctly, because is detecting that doors are still closed.
I guess you turned off your engines, without having an alternate source of power so, in the time you asked to be serviced and all the vehicles appeared, you lost your electrical or your hydraulic pressure (don't really remember which one controls the doors), so you can't open them anymore.
With the PMDG, you have to do things in the right way. As soon as you turn off your engines, connect the GPU, and activate the External Power bus OR, park with the APU on, and of course its bus connected.