That's not a GSX "bug", of course, it's a problem of the scenery you use.
In order to behave realistically, and to reduce chances of conflicts with AI airplanes, GSX vehicles will always try to use paths flagged as VEHICLE type in the scenery AFCAD, even if the route is much longer ( up to 2x longer, GSX will *still* prefer vehicle paths over taxiways )
However, if the scenery is not well made, and it doesn't include them ( all default airport have proper vehicle path, but some 3rd party airport seem to ignore this ), GSX will have to find a way to reach the parking, you surely wouldn't want it to cut through the airport just to stay away from taxiways so, if the vehicle paths are too few, not connected to the rest of the airport layout, or altogether missing, the vehicles in GSX won't have any other choice than using taxiways.
But again, that's a last resort.
A proper scenery, made with several parking spots of the VEHICLE kind, realistically placed not too far to their reference terminal, properly connected to the airport layout and able to reach any parking, using paths of the VEHICLE kind, will work as expected in GSX, and will reduce dramatically the chance of conflicts with AIs.
