1) in tutorials I have seen there is access to a menu by pressing Shift + 3 which you can use to open doors etc, I cannot get this to appear.
Which tutorials ? GSX comes with its own user manual, and nothing by default is configured to Shift+3, which is usually assigned to an instrument panel.
The default key used by GSX it's CTRL+Shift+F12 or CTRL+F12, but it can be changed so, if you saw a tutorial on Youtube, maybe that user has changed the GSX assignment (However, Shift+3 wouldn't be a very smart choice, since it's already used by FSX planes)
2) I am getting no staircases
You don't say which airplane you used so, it's not possible to be sure if the problem is that you tried to use an unsupported airplane without configuring it first. This is of course fully explained on the GSX manual: all default FSX planes and several popular 3rd party airplanes are already internally supported by GSX, but if you want to use an unsupported airplane with GSX, you must first create a configuration for it, using the airplane configuration editor in GSX.
3) What I see at the gate is nothing like the tutorials, no boarding walkways etc .. is this just because its reflecting only the 3 basic airports?
If you are on one of the 3 Trial airports (or in any FSDT or Flightbeam airport ), GSX will behave as fully registered. Registering GSX will just enable it to work everywhere else.
3) I can't open any doors except the main door although I appreciate this is probably an FSUIPC problem and not here, I have posted in their forum.
GSX is entirely unrelated with and doesn't use or require FSUIPC to operate. How you open your airplane doors is also entirely unrelated to GSX, which will simply check if they ARE open or not, but it will not try to control them: you will continue to open them as you always had to do, which changes depending on the airplane used.
Some airplanes use the same keys as the default ones, which are Shift+E and then a number from 2 to 4 after a very brief pause. Other planes have their own panel to open doors, other have options in the CDU, etc.