Welcome on the Gui3D wiki !
Gui3D is a free library for OGRE3D that make you able to build 2D menu in a 3D environment, based on Gorilla, made by betajean.
Gui3D isn't a real GUI like CEGUI or others, as it doesn't try to. It's just a bunch of widgets puts together that can be considered as a GUI.
As it's based on Gorilla, you sometimes have to deal with if you need some extras functionnalities.
Deal with Gui3D in OGRE3D is as easy as creating a SceneNode and attach a Gui3D Panel to it. The panel will be rendered as any objects in the scene ! :) See the result !