GSX has always been designed to be entirely independent of jetways, so you just disconnect the jetway yourself.
Also, there's no official Simconnect variable that we can query to KNOW if the jetway is in the retracted or extended state so, the only way we could do this, would be by using undocumented hacking.
Which we sometimes use but, since we are working on integrating GSX with SODE now, it wouldn't make much sense to invest any more developement time to work on the default jetway animations.
