Hi,
I think the best way is to have a tab system like in html. Each gui element is assigned an Id and you can in sequence get the focus on that element. The built in has something like that currently, don't know tho how it performs regards menus content and such.
Tho, I would first look at EasyGUI, it has tremendous power regards user input and can with one system handle, mouse, touch, and raycast to give total user interaction control over gui element ( and not just that, anything 3d as well!). You will anyway likely need something more "effective" than the built in gui system, and easyGUi is simply unbeatable in that field.
Bye,
Jean