Jetways customization in MSFS works completely differently than in P3D. There, we had SODE, where we could create custom jetways on the fly which were working outside the simulator default system.
In MSFS, there's no SODE, which means jetways work exactly like standard ones, so they are included in the airport file. In order to bypass the limitation of other similar jetway-replacement product, that replace the default model, with the result you'll have the same model with a single variant replaced everywhere, we supplied a set of 900 .BGL containing only jetways, based on the positions of jetways in the default scenery. This way, jetways are called by their own individual names for each gate, so we can have multiple variations of models and liveries.
The downside of this system is that's not very easy to allow user customization, because it would probably require users to recompile the airport source codes in some way, using DevMode, to create a new version of one of those 900 default airports.
About 3rd party airports, in addition to the complication of encrypted airports sold in the Markeplace which can't be read at this time (although if this will improve in the future with the new Facilities API in SU10 ), we assumed that, by now, most of the 3rd party airports already come with custom jetways, surely the payware ones do.
HOWEVER, there's another route you might undertake, and it's using MSFS DevMode to create a jetway-only airport Project! Our jetways are completely standard, which means, as long as you have GSX Pro installed, you will see GSX Jetways and Jetway links in the list of available Simobjects and Scenery in the Airport Editor, and you can place and rotate them visually.
I think this might be an attractive option for freeware users: they could easily make a "GSX enhanced" version of their airports, getting access to high quality models and lots of variations, with no effort.
