Yes… if you call addChild() then you are adding a GUI element child. If you call attachChild() then you are attaching a general Spatial just like any other JME node.
But if you want the label to pivot then I don’t understand why you are adding a node to the container. Wouldn’t you want the node to be the parent of the label? Why is the label in a container if you are going to rotate it? (This will really confuse the container.)
You will have to explain more why adding the label to a node and rotating the node isn’t possible in this case.