If you don't addChild() then the layout will be completely bypassed.
Also, it's probably a mistake that you are creating a Node, attaching the GuiControl to it, and then never actually using it.
If you already have some GuiComponents like a QuadBackgroundComponent, etc. that you know you will need then you can pass them on the constructor. Or if you know that you will have certain named layers and that they should be in a certain order then you can pass that on the other constructor. Named layers are an easy way to manage things like "border" versus "insets" versus "background" while allowing you to freely change them (by name) without having to worry about the order of the GuiComponents when you add them.
This might be helpful when understanding the component stack:
It's basically the compositional way that many GUI elements are created. For example, a Label will potentially have insets, a border, a background, a shadow layer, and a text layer. I think there is an icon layer in there, too. Any of which could be set or not depending on need. You can look into Label.java to see how it sets up the layers in its own GuiControl:
Though for including a 3D spatial in a 2D GUI, most of that stuff is probably overkill.
There was another thread where someone was creating a GuiComponent that could hold a 3D spatial and properly participate in reshape() and preferred size calculation.