Jetways in GSX are precise replacements for the default one, meaning they are inserted at the precise, location/rotation as they were in the default scenery, and they have the same overall dimensions as the default one, so you are supposed to get as good or bad result as you would have had with the default jetway, just with a much better looking model and lots of variants.
This because we can't place jetways dynamically (like we could in FSX/P3D using SODE), jetway replacements in MSFS are made with a set of 900+ custom .BGL to replace jetways on every airport in the default scenery which had jetways.
So, to be able to do this, you would need some way to edit and recompile these files.
What you CAN do, with a bit of work, is using the included Dev Mode, and create a new project yourself, using the GSX Jetways which will now will be available in the list of Simobjects that can be placed ( Bridges/Links will show in the Scenery objects list ). I guess once freeware creators realize this opportunity, we might be inundated of free add-ons with GSX, because the list of jetways that can be used in DevMode is even larger than the one used by GSX itself, for example there are multiple color variations too, and of course when making a scenery, one can decide which jetway model goes where, on each parking.
