That would explain why I was sometimes getting United and Sometimes Frontier at the Southwest gates.
That's exactly what you are supposed to see, since those operators have KDEN listed in full in the GSX vehicles SIM.CFG files, which means they both have a score of 4
Would the best option be to just add the airport code for the airlines I want to each individual file for each ground equipment, or is there a better way?
As I've said in my previous message, the atc_parking_codes of the AFCAD is used ONLY to break ties when two operators have the same score so, you can only use them to break ties between Frontier and United, you can't use them to raise score of another operator UNLESS you ALSO modify all the GSX vehicles SIM.CFG files to raise another operator (for example, Southwest) to have a score of 4 and THEN use the atc_parking_codes to break ties between it and the others with the same score.
The PaintKit documentation explains this in great detail, so it's useful even if you don't want to repaint anything, but just adjust the assignments.
Note that, whatever changes you do to the GSX vehicles CFG files, if you are not changing your own repaint, will be probably overwritten by GSX updates so, if you want a better assignment for an operator at any airport, for an operator that already comes with GSX, it's best if you just report it, and let us add it.