This has been discussed so many times on the forum, and of course it's explained in the manual. If a developer made a scenery with fake/static jetways, GSX cannot detect it. How could it, since what you are seeing it's just a piece of scenery ?
For jetways to be detected by GSX, they must be either:
- Standard FSX jetways included in the scenery AFCAD
- SODE jetways
You can customize the scenery in GSX, to TELL GSX there IS a jetway there. If won't obviously transform a static jetway into an animated one, but it will at least stop GSX from displaying front staircases and the passengers bus.