Okay, what do you mean, jetways?  Do yo mean runways that you can add to an existing airport, or maybe taxiways that you can add?  If so, you might want to get AFCAD or Flight Simulator Scenery Creator (FSSC) and fiddle around with those.  They are the primary methods of creating airports and such in the FS world.  Gmax doesn't work so well for this, as it doesn't make a hardened surface that you can land on.  It will make the textures and the objects, but they will be scenery objects, so when you touch them, your aircraft will register a crash.

Well, what you could do would be take an overhead picture of the airport you want to add the parts to and then use it as a backdrop in Gmax.  Then make the jetways how you want them and export it to FS as a seperate BGL.  That is if you REALLY want to use Gmax for this, which I would not recommend.  Too many things could go wrong.