When a scene has both the GizmoManager and the GUI3DManager instatiated with default settings, the gizmos are no longer rendered on top and instead are occluded by scene geometry. This occurs because GUI3DManager alters the
autoClearDepthAndStencil of the default utility layer.
Workaround: You can create a new utility layer to pass into GizmoManager and it will work again.
For my case, I’d rather use a new utility layer for the GUI3DManager instead, but it seems there’s no constructor option or public properties that allow you to change the utility layer for GUI3DManager away from the default