The statement ''that some third party aircraft are not compatible with GSX'' was a general statement about all third party aircraft regardless of which sim they are used on.
That wasn't very clear, especially considering is basically wrong, at least without specifying which 3rd party aircraft you are referring to.
GSX supports lots of 3rd party aircraft with its internal configurations, but that doesn't mean if an airplane is not internally supported, that it cannot be adapted to work well with GSX, provided the configuration is done correctly, and this can be done either by the user, or by the airplane developer, who might have an easier time knowing, which custom variables he used for things like doors, for example.
SOME smart developers like FS Labs, Aerosoft and Leonardo SH went the extra mile, and added their own custom coding to make using GSX even more realistic and streamlined, but the airplanes were already usable even without any special integration, like any other airplane that doesn't have any.
So no, you can't say "some third party aircraft are not compatible with GSX", because it's not really true.
Even the trickiest ones, like the Majestic Q400 that uses a custom flight and engine simulation, so GSX would normally have an hard time detecting if engines are running (for example) can be used with some workarounds like pressing Ctrl+Shift+F1 to shut down the default fuel flow the plane is not really using but GSX needs to know if the engines are stopped.
And, of course, GSX is more oriented to airliners, so it might not fit that well smaller planes like business jets or other GA, but it still can be used, just some animations might not always look right, but still you can't say is not "compatible" with these, because you can still use it.
