Red X are not a GSX problem. It's a diagnostic from SODE, which indicates a scenery that uses SODE for its own means, is not able to find objects which are created using a SODE XML file for some reason, either because they have been removed or not installed, but their SODE XML is still there.
So, if you need these objects, you must likely reinstall the scenery. If, instead, you don't need the objects, possibly because they were jetways you wanted to replace with GSX, then you must remove the SODE XML file which calls them, located here:
C:\ProgramData\12bpilot\SODE\XML