I recently installed FlyTampa EHAM v2 for p3dv5.3
When I started my sim a message appeared asking me to open the FSDT control panel and disable default jetways which I have done.
This doesn't have anything to do with 3rd party airport jetways, it only refers to the default model used in default sceneries.
At EHAM the jetways appear but don't work. GSX gives me the 'No jetways can operate at this position' error.
As you can see, both from your screenshot, and from the GSX parking page, those are NOT GSX jetways, they are made with SODE and are supplied with the scenery so, whether they work ( or not ), it doesn't have anything to do with GSX.
Do they work with using just SODE ( TAB-S ) ?
I had a look at the customisation menu and found the jetway page was empty and I can't populate it.
That's normal: when the scenery comes with its own jetways, GSX won't affect them and won't let you edit them, unless you do it by removing their SODE XML file so, you couldn't do it by accident.