After a menu unload/reload mine disappeared. This procedure restored it.
1) Issue the CUI command.
2) In the top left panel, select your (current) workspace.
3) In the top right panel, click [Customize Workspace].
4) In the top left panel, expand "Partial Customization Files"
5) Expand the appropriate partial customization file, in this case "Gizmo3D".
6) Expand the Menus section under the partial.
7) Click on the menus you want to appear, in this case Gizmo3D.
8) In the upper right panel, move the new menus up/down the list to the desired position.
9) Click the [Done] button in the upper right.
10) Click the [OK] button at the bottom.