GSX reads what are called AFCADS (that's what their called in FSX/P3D) which has all the airport data used by GSX and other addons. If you know what you're doing, you can add spots in the scenery in Developer mode but I have no idea how to do that lol.
What I do is just take the closest stand and just move it to where I want the parking spot to be. For example, RomanDesign's Ottawa, I operate the iniBuilds A310 often doing RCAF heavy transport runs. Near a hanger is where they service the aircraft and there is no parking spot there. I just moved the parking spot that was inside the hanger to where I wanted it and set everything how I like it and it works perfectly. Only issue is you can't spawn directly into that custom spot. You would have to either slew, or use GSX to teleport yourself there.