Switching GUI on and off

you can use bool values as you said. you can make the gameobject that has the gui active/inactive too. use the active property. also you can enable/disable the component that has the gui with enabled property. the unity GUI system is a low level system. you can create classes for GUI and expose properties like visible for them and use things like this

if (visible == true)
{
//call the GUI method like GUI.Button ()
}

also you can expose other properties like letf/top and width/height and value and ... a GUI Element class would look like

class GUIElement
{
public bool visible;
public float left,top,width,height;
public string name;
GUIElement (string s)
{
name = s;
}
virtual Draw ()
}

then you should have a form class that you add other elements to it and it will call the draw method of all gui elements inside it in it's onGUI

class form : MonoBehaviour { private List elements = new List();

void OnGUI () { foreach (GUIElement g in elements) { g.Draw(); } } } then you should define classes for Button and TextField and ... that they are children of GUIElement and define their own Draw methods that use the properties to draw the control a draw method for a button would use a GUI command like this

if (GUI.Button (new Rect(left,top,width,height),name))
{
}

you can have a deligate to call when this happens and also form class can have methods for creating a button and label and adding the created element to the list

you can create XML file structures to read and create GUI based on them and many more