GSX doesn't create SODE jetways on 3rd party airports unless you explicitly enable the checkmark on the "Jetway exclude radius". Then, assuming this was your intention, you must exit the sim, and use the GSX Control Panel utility to compile the created Exclude file, which will exclude all the jetways that had their Exclude option enabled in the GSX customization editor.
If this wasn't your intention, and you only wanted to disable the rear staircase, you only had to check the "Disable staircases" option, and don't enable the Jetway exclude option.