That's normal, since are the ones we assigned to KSEA.
The parking codes in the AFCAD are used ONLY used to "boost" a parking preference, if the operator IS already assigned to GSX, and of course is correct, because atc parking codes in the AFCAD contains many more airlines than the one that operate ground services at an airport, since they include also airline that "just" fly there, so it would be wrong if GSX would display them all.
The PainKit documentation explains in detail how the scoring system works, and how you can change it.
And besides, what ground operators you think are missing at KSEA ?