How the message looks like ? It's in the green top bar, or in the GSX menu while selecting a parking ?
If you don't see any parkings in the GSX parking selection menu, maybe there aren't any parkings large enough for the airplane you have loaded, which of course is not a GSX problem, which reads the parking size from the scenery AFCAD, but you can override the size/max wingspan from the GSX parking customization page.
If, instead, it's a message of the top green text line, try a "Restart and rebuild the airport cache" command from the Couatl menu. And be sure GSX is really reading the correct AFCAD.
