Hi Nantuket
There is no difference between FSX and Prepar in the way Native and 3rd Party airports work. GSX is even better (especially Level 2) with Prepar, it has now PBR and new great animations. Just be sure to use the newest Prepar 4.5 from the beginning, it will save you some troubles with upgrading later. As far as I know you don't even have to buy GSX again. Just download the GSX Installer for Prepar 3d and you can use the same Activation you had. (At least it worked all that way for my airports, FSDT is very generous in crossgrading on a new plattform).
Custiomization works the same, you have to customize 3rd party airports yourself. But you can do costomization for every kind of airport. I don't know ORBX very well, I just heard that sometimes the Jetways are modelled static and hard to get rid of, but that is no limitation of GSX but of the scenery itself.
Hope this helps.