hmm, what about reading the correct "streets" from the corresponding bgl files ?
What do you mean with "streets" ? Vector streets are only for the road traffic, and are usually outside the airport, so GSX won't use them, and it would be wrong if it did.
For ground vehicles at airports, there's a PROPER kind of path, which is NOT a taxiway, and it's the "vehicle" path, which is displayed in pink in ADE. GSX vehicles will use those by preference, giving them more weight so, even if the path is longer than taking a taxiway, they will still prefer the vehicle paths.
Obviously, if the scenery AFCAD doesn't contain any of these paths, or they are not connected correctly, or there aren't any close to the parking you are being serviced on, GSX should still trying to use SOME kind of path, otherwise you'll see vehicles cutting through grass to reach you directly. So, when nothing else is available, GSX will use taxiways, and even runways, even not even a taxiway can be used.