So it looks like that you have to modify the parking slots of any Addon scenery the first time you use it
That's to be expected, if the Addon scenery doesn't have any customization already. GSX doesn't have any notion of the outside world, except from the AFCAD, the airport customization is a way to give it more information, that can't be taken from the AFCAD.
A conflict with a pushback truck is a special case though:
if it was the default Pushback truck, the GSX installer should have disabled it BUT, only if you use the default airport vehicles .BGL. If you installed a 3rd party airport vehicles replacement set, the default pushback won't be removed anymore. We are working on an improved installer that can disable the pushback in any case, even with using a set of replacement vehicles.
If, instead, that pushback truck was an object added by the scenery itself, there's no way around than doing a customization for that parking.