Google Cardboard is no longer directly supported by Unity. If you have any issues related to Google’s latest Cardboard XR plugin for Unity, the Google team has requested you open issues on Github here where their team will address.
Thanks, as a Unity subscriber I thought asking other users how they would use Unity to create a working element (Gaze reticle) in an app produced in Unity, was appropriate. It might be a feature request for the Google team, but is something that could be created in Unity too.