If you want to *change* the what GSX by default assign to a parking spot, this is explained in the PaintKit documentation. Even if you don't want to repaint, the documentation should be read to understand how the operator assignment logic works.
If, instead, you only want to *select* an operator amongst all the valid candidates GSX has assigned to an airport, you can do this with the latest Couatl 3 Public Beta or the latest GSX Public Beta, available in the "GSX Backdoor" forum, because there's a new selection menu that will allow you to do that.