Is GSX required to have the features of all the vehicles and people in the "Busy Hub" video?
No, it's not required, but those vehicles and people are of course handled by the Python scripts, to manage all the logic to have them appear/disappear in smart ways in order to save fps. The scenery simply wouldn't fit in memory if we just put "everything in" with normal SDK methods. So it obviously requires the Couatl engine, like all our other sceneries, which will be automatically installed.
GSX is required just for the docking systems, and the marshallers, of course. However, since it's free to use at our airports, it's not really an issue, even for those not having purchased it yet: the Trial version will behave like a full version on any FSDT/Flightbeam airport, so it's just a matter of downloading it.