Tried it now, using the current 1.1 KDEN installer, the AFCAD lists Gate C41 to have ONLY an SWA code so, although AA, United, Jetblue, Delta, Southwest and Frontier are all allowed at the airport with an equal score of 4, that gate should only get Southwest, because that's the only airline code that parking has.
Tried in GSX, with the PMDG 737, went to C41, called Deboarding 10 times in a row, and I got Southwest every time, and of course all the equipment was entirely Southwest.
Just to be sure, I've edited the AFCAD to add the FFT airline code to C41 and made the same test, over 10 Deboarding requests, I've got Southwest for 5 times, and Frontier for 5 times, which is exactly what it's supposed to happen. Never got mixed vehicles in any of the tries.