This one shouldn’t be too hard to fix as all of the needed underpinnings are there and this is (somewhat) documented in the SDK.
I have tidied up the livery packages in my community folder and every single one of them has correct atc_parking_codes and icao_airline. (These liveries consist of A20N, A321, and A320, latter two being two copies of the Canadian Mods A321 with different icao_model in aircraft.cfg).
I have noticed that sometimes, this results in real world flights showing in the correct livery! Nice! But sometimes it’s a random livery like a FFT A20N wearing a VOI paint. That’s a separate issue (or maybe it’s not). Anyway:
I created a test airport which has all gates set up with a single airline in airlineCodes. The expected result is nobody parks there if they’re not listed. You can inspect the XML at Test airlineCodes - Pastebin.com and tell me if my test is invalid. I tried putting the same airlineCodes on all 13 gates, making them all SWA, or all DAL, and the liveries I would get would be random. Once or twice I got something that seemed semi-plausible, like for all-DAL gates I got one DAL, one default Asobo Neo, one Generic Twin, but it was not repeatable and the next time VOI and SWA would park in DAL gates.
Obviously it’s important to be able to make gates A, B, C at Atlanta, B, C at ORD, or A, B at MDW reflect the dominant airline in those gate groups. My hope is this is addressed before a bunch of scenery gets developed and released with no airlineCodes set up in them, because once those payware makers get your money they don’t want to go back and fix it.