I'm sure it is discussed before but it seems in JFK default vehicles around airport are hidding Marshall / conflicting GSX units. How do I remove them? 
What do you mean with JFK ? Default JFK, FSDT JFK or FSDT JFK V2 ? When you activate GSX at any gate, it will remove automatically ALL default vehicles there, and this is the normal behavior with a default scenery.
With FSDT JFK, you are not even supposed to have default vehicles, because the scenery has all AFCAD parking spots labeled as GA, so no default vehicles will appear there, and the scenery comes with a GSX customization file, which is supposed to position all GSX objects in a way they won't conflict with any other scenery object.
If you use FSDT JFK, are you maybe using a different AFCAD ? If you don't use the AFCAD from FSDT, you'll lose also the GSX customization, and that might also explain why you see default vehicles.
Also, on a default airport, that has default vehicles, while GSX will remove them at a parking automatically when you select it, if you use Time Acceleration higher than 4x, then go back to 1x, they will re-appear, but GSX won't remove it again in that case.
