GUI objects are used to create dialogues and other interface elements following user's gaze.
Such objects always follow the user's gaze using tag along effect on HoloLens and other HMDs. On phones and tablets GUI objects maintain static position in relation to the device screen. Thus GUI objects is a great tool to create overlays on such devices.
The distance to the user is automatically adjusted so that the GUI object is as big as possible while still fiting into the screen/FOV.
To turn an object into a GUI object, rename it:
gui: <gui object name>
gui: Text Alert1
You can also create an empty GUI object and place objects with geometry inside it. In this case all the nested objects will behave as a single GUI object.