GSX doesn't replace any docking systems found at 3rd party airports. In fact, if an airport comes with its own docking system, you'll want to customize that parking using the GSX parking customization page, to disable the GSX marshaller at that spot, otherwise you would see both.
GSX docking systems are found only at FSDT airports.
We could add an option to place GSX docking systems everywhere, but it would work best only at default airports or 3rd party airports that don't have a docking system already. The issue is similar to jetways: we would need a way to exclude the 3rd party docking system, but the scenery should be already made to have docking systems as a separate object AND we would have to support each airport individually so, similar as the jetway issue, we would have to start charging per-airport, which is not something we want to do, because the one-time purchase is undoubtedly the feature that most contributed to GSX success so far.
