Nothing has changed.
Since GSX has been released, the atc_parking_codes of the airplane.cfg controls ONLY the livery of the ULD containers, while the parking codes of the *scenery* controls the GSX operator for all the vehicles, if the match their own atc_parking_codes in their own SIM.CFG, according to the normal scoring rules, which can be overriden by airline code field in the GSX parking customization page.
As explained on the manual, and on the forum, so many times, it would be wrong if the atc_parking_codes of the airplane.cfg controlled the ground operator, because it would mean your own ground crew would be able to follow you everywhere you go, which is not real.
And, it would be wrong too, if the airline codes of the *parking* would control the ULD containers, because if they did, it would likely result in ULDs unloaded at the arrival of a different kind of the ones loaded on departure, in the likely case the airline codes of the *scenery* would be different.
That's why, GSX correctly works as it always did:
- atc_parking_codes in the airplane.cfg controlling ONLY the ULDs
- parking codes in the scenery controlling the vehicles, according to the scoring between the code of the scenery and the atc_parking_codes in the SIM.CFG of the vehicles, with the ability to override them using the "airline code" field in the GSX parking customization page.