If I define these in an a AFD file, will GSX follow these lines?
Absolutely, and in fact it's best if your AFCAD has them, because it minimize the chance of clashing between ground vehicles and AI. GSX vehicles give an higher preference to vehicle paths, even if the path using taxiways would have been shorter.
Similarly, vehicles that are not tied to a parking ( Follow Me, Catering and Refuel tank) are created on a parking of the vehicle type, but if there are none, or the closest one is too far from your place, they'll start on a regular parking.
So, a best practice would be putting enough vehicle paths to allow vehicles to reach any parking, and some parkings of the vehicle and fuel type, spread apart around the airport.