The problem might be caused by another scenery in conflict that has changed the ground elevation, but it might also be a 3rd party mesh. We usually indicate OrbX, only because they are the most common cause of the problem, but anything that changes the ground altitude might cause this.
GSX has a way to fix these problems, by entering the parking customization page and clicking on the small airplane icon where it says "Ground Elevation", this will read the current elevation from the sim, and would fix the floating jetways.