I am trying to set my gizmo origin point to the center of one of my sub tools because it is way off screen.
In Gizmo/move mode, there are 6 icons at top. Third from left is “go to unmasked mesh center”.
Also, be sure to upgrade to the most recent version of Zbrush. It is free for registered users.
How to upgrade: https://support.pixologic.com/Knowledgebase/Article/View/294/84/upgrading-from-an-earlier-zbrush-version
Make sure “activate symmetry” is turned OFF before centering gizmo to mesh.
Also if you press alt over the rotate icon it will zero out your rotation on the gizmo and alt+home (the house) will center gizmo to world space. Very helpful!