There are several reasons why you might not be able to view the GUI in the list:
1: You do not have it packaged with a server/client.cs a description, and the GUI file, you need to have the server/client.cs exec the GUI file, Ill put this in the guide.
2: You have a server.cs file that does not execute the GUI until you load it up in game, load up a mission with it enabled, then open the GUI editor
3: Your server/client.cs or GUI file syntaxes and the engine cannot load up the GUI.
Those are probable reasons.
In your server/client.cs file you must have:
exec("./guiname.GUI");
That will make the engine execute the GUI file.