Nothing related to GSX, of course.
It's an automatic optimization made independently by FSX for EVERY 3d object, in order to gain some fps, FSX tries to decide if an objects is inside your field of view, and it does it very aggressively ( too much, probably ), because the whole object will be culled even if only part of it is outside your field of view. It doesn't do it for GSX vehicles, every 3d objects is affected, for example default and custom jetways have exactly the same behaviour, it's just that with GSX installed, you are more likely to use closer viewing angles, that shows this more easily, but it has always been there.
