That's not a GSX problem but, instead, a problem of the scenery you are using, which is not made correctly.
If the scenery AFCAD is made in a realistic way, with proper dedicated vehicle paths and proper multiple vehicles parking spots, which would (as in real life) allow vehicles to reach any parking without crossing taxiway, GSX vehicles will use them by preference, even if the route is longer, since they are being assigned more weight in the pathfinding algorithm.
But if no such paths exists, GSX vehicles must still use some kind of path to reach your parking (otherwise they would cross through grass), so they don't have any other choice than using taxiways or even runways.