When you attempt to use GuiService’s AddSelectionParent function on a BillboardGui it throws an error saying that the object is not a GuiObject.
BillboardGuis don’t inherit from GuiObject, so I’d say this is intentional. BillboardGuis and ScreenGuis are containers and the AddSelectionParent function expects objects like TextButton, ImageLabel, Frame, etc…
I see. I didn’t really think of it this way.