I also have the same problem with some of my payware airports, especially Imaginesim KDEN v2 (FSX). Both stairs and coming up at all of the gates with jetways.
I'm not sure if by "the same problem" you are referring to the inability to see the "customize this parking position" menu option at that airport, or just the fact (as you are describing it) that all stairs comes at gate with jetways.
Because, as explained many times already (and it's clearly explained in the GSX manual too), if the scenery doesn't have real jetways, GSX can't know there is a jetway there, which is why we have the option to customize a parking position in the first place.
So, you have to customize it and, this thread is about an user not seeing the option to customize a parking on a specific airport so please, be more specific and confirm if this is your case too, or you just reporting having stairs on parkings with a jetways on 3rd party sceneries with fake jetways, because this has to be expected.
And a side note= My PMDG 737NGX, in PMDG's Southwest Colors, when parked at gates at Concourse C (SW), only Frontier service vehicles appear. How to fix this? I'm not sure but I think the Frontier service vehicles are there for all planes i.e. United, American etc. Maybe Frontier is the only one activated in Denver?
As explained in the GSX manual, vehicle assignments depends both on airport codes AND the parking codes. If two companies are assigned to an airport, but the parking code of the scenery has a code for a company but not the other, the one with a matching parking code will always get that parking. If both have a parking code, the selection will be random again.