How to use the GUI layer?
The GUI layer is a kind of “window glass” fixed right in front of the camera. Things drawn in the GUI layer appear over all 3D objects in the scene (exactly like they were drawn in front of the camera). You can’t draw anything you want on it, only the components GUIText and GUITexture. There is also a new GUI system including more dialog related itens (GUI.Label, GUI.Button etc.) which “kind of” draws in the GUILayer (the docs say GUI system is not tied to the GUILayer).
If you want to create a health bar, for instance, GUITexture is the candidate; if you want to place text displays, GUIText is easier. If you want to create buttons, menus, text fields and other visual controls, the new GUI system is what you need.
You can read about these componentes in: