I've noticed on some of the Files made by Cartyana he says about setting the XML File for stands or Jetways to OFF?
A Red X is a message from SODE, telling a scenery has a SODE XML file ACTIVE ( indicating you haven't disabled it ), but SODE can't find the SimObjects that are supposed to be created by it.
It doesn't have anything to do with GSX because, if GSX had a problem, you wouldn't see any Red X, since GSX creates its own jetways so, if they were missing or the GSX customization file called from specific models you don't have, you won't see a "Red X", but an error about a specific object name not being found from GSX.
If you disable the 3rd party scenery SODE XML file correctly, the only thing that is supposed to happen, is the SODE objects would just disappear. If you see the "Red X" it means the opposite: you haven't disabled the SODE XML file, but for some reason the objects that belong to that scenery can't be found, this is usually a problem with that scenery installation, but it's impossible to provide a generic suggestion, because each scenery has its ways to reinstall things, especially when SODE jetways are concerned, some products might have configuration utilities that switch between SODE or Standard jetways, but it's not a rule whether they also install/remove the related 3d models for the jetways too, they might or they might require reinstalling the scenery.
Of course, if you disable the SODE XML file, if the original SODE jetways are not installed ( this is the meaning of the Red Xs in SODE when the XML file is ACTIVE ), it won't be a problem anymore, since you are going to use the GSX ones anyway.
So, it seems your problem it's just you haven't removed or renamed the correct XML files for those sceneries. I guess Cartayna suggestion is to RENAME the XML file to .OFF ( = replace the .XML extension with .OFF extension ), instead of removing it, so it might be easier to get it back for any reason.