CardbouardGUI

I am relatively new to Unity3D. I am converting a existing game to VR using cardboard SDK.
We have added network HUD which displays UI using onGUI calls which display normal UI even in stereo mode. When I added CardboardGUI prefab to onGUI(and made changes to onGUI callback), I don't see UI displaying anywhere. Can someone please guide me here, Am I missing something?

Thanks & Regards,
Pradeep