While in demo version I noticed a few cargo and baggage loaders working with my ai planes, so I assumed some vehicles would work.
You just saw the default ground service vehicles, which works with AI airplanes.
I did end up buying GSX because some is better than none but after buying GSX I see that it doesn't work at all with ai planes.
There's no difference between Trial and purchase version of GSX. GSX vehicles service only the USER airplane. It's just that you confused the default ground service vehicles ( that were always there in the sim ), as something that was was "added" by GSX, but it wasn't.
I have now reluctantly uninstalled GSX and the money I paid for it
That doesn't make any sense: the most important airplane in the sim it's YOURS, and you uninstalled GSX because it only *greatly* improves only your own airplane ?
but I have also lost those P3D default vehicles.
GSX doesn't touch the default vehicles and doesn't have any effect on them, except the default Pushback truck, which is removed when GSX is installed, and restored when GSX is uninstalled.
Can you help me figure out what might have happened
You probably put the ground traffic density slider too low. Nothing related to GSX.
Is there any way to have GSX work with ai planes even if I have to alter something?
No, there isn't.
the GSX vehicles look so nice
Of course they do. And the obvious reason why they don't service AI airplanes, it's BECAUSE they are "nicer". Which means MORE complex and fps and memory consuming. There's no free lunch. If you add something "nicer", you pay for it with performances and fps loss.
If the "nicer" GSX vehicles were able to service AI, the fps will collapse, assuming the sim would even RUN, since it will probably OOM ( Out of Memory, since it's a 32 bit app that cannot allocate more than 4GB or RAM, regardless how much RAM you have ) before giving a chance to the fps to go to zero, which is what would probably happen if the sim was a 64 bit app instead...
and it's a shame that I can't enjoy them.
Yes, you can. On the most important airplane of the sim. Your own.