Quick question : what does it mean to say that an aircraft is "supported" by GSX ?
That we already made a GSX configuration for it, which is internal to the program, so you don't have to create one yourself.
If you want to use an unsupported airplane, you must first create a GSX configuration for it, using the internal airplane configuration editor. Once you do that (the procedure is fully explained in the manual), the airplane becomes "supported".
I have to ask because I tried to set the jetway (Ctr J) to the CS 757-200 at gate 14, 11 and 34 and it never came properly to this aircraft. Should I fine tune it with custom setting etc. ?
Jetways don't have anything to do with GSX, that doesn't control, doesn't affect, is not affected by jetways. Nothing you do in GSX has an effect on jetways in any way. They ARE bugged, and there's nothing you can do to fix them completely.
This will likely change in future versions of GSX, but it will still require all sceneries that use default FSX jetways, to be updated.