Actually, when you are using UnityGUI (the standard way of implementing GUIs in Unity), it is not related to any camera at all. It's actually drawn after all cameras have rendered.
GUI.depth is only for the depth inside the GUI-framework; so that defines in which order the GUI objects are drawn but it's not related to the cameras.
You might have to consider using a gameobject based GUI system (like e.g. EZ GUI) to accomplish what you want.
Can u provide some more information and explanation . How to draw 3d object on a screen which is full of GUI.DrawTexture . GUI.DrawTexture is covering the entire screen and on top of that screen i want to draw 3D object .