Have a look at the SIM.CFG of one ground operator object in GSX, for example this one:
FSX\Simobjects\Misc\FSDT_Staircase\SIM.CFG
Look all the [fltsim.XX] sections, to see which operators are available in GSX, and on each one, this line:
couatl.atc_parking_codes =
Will tell you which code is linked to the operator.
You can place this code in the GSX parking customization page, to override the default settings for that airport, and you can do it for a single parking, a terminal or the whole airport, depending on what you select on the left side "tree" list.