guiText is the old gui system that is not part of the OnGUI stuff. guiText is a gameobject that you move forward/back in depth using its transform.z position, like any other gameobject.
Stuff inside OnGUI will always be drawn over the viewport.
GUISkins are limited to using mono-colored TTF fonts, though. If you need to control how stuff is layered, like Seon said, you can’t mix OnGUI with GUIText…use one or the other.