You don't say which "MMMX" you are referring to, so I must cover both cases:
- If it's a 3rd party airport, you MUST Disable GSX replacement jetway files for that airport, because those are only meant to be used for the default airport. If you don't do this, you can be sure Jetways won't work.
- If the airport is default, either the GSX replacement jetways or the default jetways should work the same, but if they don't, it's NOTHING that is related to GSX. Nothing in GSX controls how the jetway is moving or docking or not. The only thing that controls if a jetway works or not is how it's modeled, how is placed, the airplane you are using, and it's parked in relationship to the jetway. GSX only sends a command to operate the jetway, but the rest is completely up to the sim.
And yes, it's entirely normal GSX will always says "default jetway" in MSFS, because it's the only jetway supported.
Default means "using the standard SDK method", it has no relationship how the jetway looks, it only refers to the way the jetway works, and in MSFS we only have default jetways, opposite to FSX/P3D, where you might have other systems like SODE, which GSX supported.
