You can't.
The vehicles you see when starting a flight are not GSX vehicles. Sure, this might sound strange to say, because they look like GSX vehicles because, of course, GSX replaced default MSFS ground vehicles too but, for all intent and purpose, they still are default ground vehicles, which are controlled entirely by the simulator, which doesn't know anything about your GSX customization.
After you start GSX and select a gate, those vehicles will be replaced by proper GSX vehicles ( again, looking the same, but controlled by GSX ), so they'll appear according to your customization.