The way GSX Jetway replacement is done in GSX is completely different than P3D. There, we had SODE, which allowed to create custom Jetways on the fly, completely bypassing the default Jetway system in the sim.
In MSFS, since there's no SODE, we use the default Jetway system, and provided GSX with a set of replacement files that has been pre-built based on default airports data, so they can't be edited like in FSX/P3D.
With 3rd party sceneries, the issue is even more tricky, because possible legal issues would come into play because, to do something like you are asking (adjusting a 3rd party jetway in a 3rd party scenery) would require creating a new adjustment scenery, which would involve decompiling and recompiling something based on 3rd party data, something that even if it might technically legal (and not everywhere, Europe has different regulations about decompiling than the US, for example), it might not something 3rd party developers might be happy with.
