Even if you see the parking spots in GSX ( because of the profile ), it doesn't mean they are usable, because in order to be usable ( by GSX and, in general, the sim ), a parking spot must be connected to the rest of the taxiway/paths layout, since every vehicle needs to use those connections to work properly, and this is valid for Pushback, Catering, Fuel, Follow Me and Deicing, that all need to use the actual network of nodes and path in the scenery to work, all connected to all parking spots.
That's why GSX warns against using isolated parking spots, and that's why the simulator own DevMode editor won't even compile an isolated parking spot anymore, because those will cause all sort of problems to the default AI system too.
In fact, if you open the GSX airport customization page when a profile with mismatched parking spots, it will warn you of the problem, suggesting to create a new "clean" profile, with only parking spots that matches the airport in the sim, and this is the best course of action, so what you'll see in GSX will match what's in the simulator.
Before the Navdata API was introduced, it was possible to put a dedicated .BGL file in the GSX airport profiles folder, which would be used only by GSX, but this was basically a temporary way to have Marketplace airports working, before we had any way to read them, and required to obtain an unencrypted version of the same airport from the developer, but this works only with the legacy "airport cache" method, because when the Navdata API is in use, no .BGL is ever used by GSX and all data comes directly from Simconnect.
While with SU10, we enabled the Navdata only optionally ( since it added extra issues with Jetways ), starting from SU12, we enable it by default. However, it's still available, by editing the Couatl_MSFS.INI file as explained here:
https://www.fsdreamteam.com/forum/index.php/topic,29560.msg191813.html#msg191813This assumes you can obtain the unencrypted version of the scenery from FlyTampa, one that contains all parking spots, which was the one the GSX profile was made for.
The root cause of the problem seems to be the scenery is likely coming in two different configurations: one with all parking spots as in real life, and another one with static airplanes, so all parking spots assigned to them has been removed, so they won't conflict with AI, and the Marketplace version seems to be the one with static airplanes, which would explain why lots of parking spots are missing.
This would have been a problem even if you didn't had GSX: if the online controller directed you to a parking that doesn't exists in the scenery, you would have found a static plane there!
This, assuming they didn't do a mixup of files, and released it on the Marketplace the version of the airport file made for static planes, without including the static planes. In fact, I find strange you were assigned to a parking that has a static, and didn't notice it was the main problem there, regardless of GSX, so maybe that's what happened, could be the Marketplace version has the reduced version of the scenery, but without the static planes that would require that file to be used ?