Whenever you see missing parts of objects, it's never a problem of GSX or a scenery, and it's *always* a problem of a tweak that you shouldn't apply. Fact you only see this with GSX, only means you don't have other scenery objects that use the same materials/shaders (which are of course fully standard) that in your case are exposing the problem caused by some tweak.
I'm not sure if the issue in your case is that specific tweak you reported, because tweaks that work on a system might not work on another one, that's why they are not documented in the first place but, first of all, reset the FSX.CFG to default ( move it to another folder, so the sim will create a new one with all default settings ), and apply ONLY the HIGHMEMFIX=1, which is the only tweak that is absolutely required.
Note that, you might want to save all the lines containing this:
Simobjectpaths=
from your old FSX.CFG file, otherwise you'll have to reinstall several addons (GSX, SODE, and others), because they won't work without their lines added to the simobjects search paths.
