Yes I do use FSX and I did not have this problem before I upgraded to GSX level 2.
Well, of course you haven't, because the old jetway model was different.
As I've said, the default scenery flickers in a lot of places (just not on ground markings), but elsewhere and, since the jetway replacement model appears on default sceneries, I wonder why you ntice only the GSX L2 markings, and not the rest of the default scenery, which flickers too. Because it is an FSX problem that has always been there.
As I've said, you can of course decide not to use ground markings, if you customize the jetway.
Of course, in P3D4 there's not flickering whatsoever, because we tuned the objects for P3D4, and it wouldn't be right to penalized P3D4 users, if we raised the ground markings just for FSX, causing an annoying visual effect of raised textures on P3D4 so, when we have to choose, we give preference to the solution that looks best on P3D4.