In the GSX Customize Airport Positions I changed the code from DAL to AAL but Delta planes still park at these gates.
You misunderstood what the airline code does. It's not affecting the original scenery AFCAD, hence affecting the AI airplanes. It's something that only GSX reads, and it's used to override the airline code
used by GSX vehicles, to drive the choice of the GSX ground operator regardless of what's inside the scenery AFCAD.
It won't affect AIs in any way (the sim AI engine doesn't know anything about the GSX customization). If you want to change the AI parking assignments, you must change the actual scenery AFCAD, using ADE.