The manual encourages to download and install GSX, because if you don't, you'll won't have docking systems. As explained on the manual and on this site, GSX will work IN FULL at all FSDT airports, no limitations, so there's no reason to download it and install it, regardless if you intend to purchase it or not.
Purchasing GSX will allow you to use it at all the remaining FSX airports.
Just download and install the GSX installer from the "Products" page of the site. If will work as a registered product, at JFK at all the other FSDT airports.
