If I am flying a United 737 and the aircraft config has atc_parking_codes=UAL
As I've said in my previous message, the codes of the aircraft you are flying DO NOT have any effect on the Ground Operator, for the reasons I think to have explained quite throughly in my previous post.
the AFCAD has UAL as the gate, and GFX has UAL in it's configuration.
That's another issue entirely, and even if it might sound confusing, it all makes sense, and it's explained in the manual and, more exhaustively, in the GSX PaintKit documentation.
The parking codes in the AFCAD are NOT enough to have an operator assigned there. The operator must be first assigned by GSX to that airport, and it will obtain a certain score, depending how many characters from the
icao_prefixes in the SIM.CFG of the GSX Vehicles match the ICAO code.
The parking code in the AFCAD will be used ONLY when breaking ties, when there are two or more operators with the same score. So, in you case, the issue is that UAL is not assigned to this airport by default. IF UAL was assigned to this airport
and had the same score as the other two (Delta and Frontier), the UAL parking code would
tip the balance in favor of United Airlines. GSX, in this case, assumed this IS an airport that United operates from, and this IS one of their parking, so no other operators will go there.
The reason for this is, again, to prevent unrealistic situations of having your ground crew following you everywhere in the world, for example at places where United has no base of operations, but it might still have its parking codes in the AFCAD, just because it goes there (outside the US, for example) or it does *code-sharing* with a different airline. This would happen if GSX "just" read the AFCAD parking codes instead of correctly doing what is doing now, which is using them only to break ties amongst operators that has been assigned to the airport, at the same score.
However, you CAN change the assignment without having to edit all the SIM.CFG of all the GSX vehicles: just
rewrite the airline codes in the GSX editing field, and
Apply the changes. Since YOU made the edit, GSX is assuming you know what you are doing, and you know United IS an operator there, so it will ignore its own internal assignment, and just trust what you said.
And of course, you can change multiple parkings at once with a single edit, for example setting a whole terminal to a single (or multiple) airlines, just by selecting more than a parking in the left-hand side of the editing window.