Well this isn't really a GDScript thing per se. It's more of a problem solving thing. You need to figure out how the GUI should work in general, then after that, figure out how it can be done in GDScript, or any other language for that matter.
In this case, all you need is to move the camera to certain places whenever you touch a button, using their signals as you normally would. For example, when the "settings" button is pressed, then move the camera to some other location.
As for having 3D GUI elements, you would need a viewport to render the GUI into 3D. This demo should show you how it's done.