Up to now, all assets you attached to the guiNode were two-dimensional, such as fonts and icons. What happens if you attach a 3D object, such as the blue cube geom, the blue cube, to the guiNode instead of the rootNode? Why not try and see yourself.
Modify a fresh BasicGame and add the following change:
geom.setLocalTranslation( settings.getWidth()/2, settings.getHeight()/2, 0); // center the box geom.scale(10f); // scale the box guiNode.attachChild(geom);
When you run the sample code, it seems the jMonkeyEngine renders the box as before. But when you move the mouse, the box does not turn along as objects attached to the rootNode would.