There's a complete user manual which can be accessible either from the Addon Manager menu -> "Open documentations" button or outside FSX, in the FSX\fsdreamteam\Couatl\GSX folder
In any case, GSX doesn't change anything to jetways behavior: you still call them as before, using CTRL+J, assuming of course the scenery comes with animated jetways, like all default sceneries, all FSDT sceneries, most of the UK2000 and latest Flightbeam KPHX.