I would like to manually set the default spawn point for buses, baggage trucks, catering and so on at the airport.
As explained in the manual, and as discussed so many times in the forum, GSX will use the nearest parking of the Vehicle type for vehicles, and the nearest parking of the Fuel type, for fuel trucks.
So, if the AFCAD of the scenery is made correctly, they will spawn when they are supposed to.
If the AFCAD of the scenery is not made correctly, lacking these, a random parking will be used instead so, you can use the free ADE utility to add these points, doing exactly what you are asking.
Could be in the customize airport position dialog.
No need to duplicate in GSX something that is already possible to do with a very good freeware utility.
Even the spawn point for people when you dont need the buses in those very small aiports.
This has also discussed so many times on the forum, even very recently, for example here:
http://www.fsdreamteam.com/forum/index.php/topic,20310.msg140133.html#msg140133