But before SU10 the gsx custom jetways worked at levc(3rd party airport), and now they don’t work, that’s the question
The issue is that you should NOT use GSX replacement jetways at 3rd party airports because, if they they were "visible", they wouldn't have worked very well anyway, unless in the very unlikely case of the 3rd party airport matching the default airport jetway locations exactly ( both position and gate they are associated to ), and this would have caused all sort of bugs that users mistakenly associate with GSX, like passengers not showing or walking in the wrong place, which most of the time are caused precisely because GSX couldn't detect the jetway properly, because you tried to use the GSX replacement file on a 3rd party airport when you weren't supposed to.
So, please, use GSX as expected, and disable jetways at all 3rd party airports using the "Disable 3rd party button", which should always be used when you add a new 3rd party scenery, and if the airport is not recognized automatically, because the scenery doesn't confirm to the official package naming convention, add the ICAO manually to the Disabled list.