You won't see any FPS hit with GSX because it only spawns objects for your parking and doesn't spawn anything for AI.
Also, the stuff like the fuel truck and luggage only works when you're parked, so even it it did cost FPS you wound notice it because the plane is parked and not moving. Another thing is that the objects won't spawn until you land and select the gate. So even if there was an FPS hit from that you'd only be taxiing at that point and would likely not notice.
If anything, the small amount of objects that GSX places would cost about the same as one AI plane at an airport, which is basically nothing.
Since GSX offers a free trial and works for free at any FSDT airport, just download it and try it out and see for yourself.