All GSX vehicles will use vehicle paths by preference, IF they are available. Taxiways will be used ONLY if there are no proper vehicle (not roads) paths available in the AFCAD.
A mistake some scenery developers do, is to add vehicle paths but not connected to anything.
A vehicle path should at least connect to a least one node that can reach your parking: if they are totally separate from the rest of the network layout, they won't be used. All vehicle paths in default airports always have a least one point where the path connects to an apron node.