GSX can only rely on what information the scenery developer has placed in the AFCAD file.
When vehicles are concerned, GSX will TRY to use paths of the "Vehicle" type by preference but, if there are none, there's no other choice than using taxiways.
The default scenery for Copenhagen has proper vehicle paths included so, I must assume that, if you see GSSX vehicles using taxiways, maybe you are using the FlyTampa version.
And yes, checking the AFCAD made by FlyTampa, they haven't included any dedicated vehicle paths, so it's entirely normal that GSX would use taxiways, since there is no better choice in the scenery.
Of course, I guess that nothing would stop you from adding the missing vehicle paths yourself, using any of the available airport editing utilities, like ADE or AFX.
