As explained on the GSX manual, and as discussed many times on the forum, GSX doesn't have anything to do with jetways, doesn't change them, doesn't operate them and doesn't nothing to improve them.
Installing GSX won't change anything how they would work on any given scenery, since GSX doesn't make any changes to them, so it's sure that nothing in your scenery has changed in relationship to jetways, after installing GSX.
Since jetways operates entirely separate from GSX, nothing you do in the GSX customization will have any effect on them. The default jetways are NOT perfect, and you should expect bugs.
This, of course, will change when we'll release the next GSX version with SODE integration, but the first scenery to use them will be KMEM so, it will be a while before we'll be able to retro-fit all our previous sceneries to use the better jetways offered by SODE, but we'll get there.