Any ideas on why the "turn on notifications for this gallery" is not appearing in the options widget?
The Media Gallery - Links widget provides a Subscription link only when rendered on the Media Gallery files directly and will only subscribe the accessing user to comments on that one file.
Here is a sample from my local instance:
If the intent is to subscribe to the actual gallery that is unfortunately not possible in any currently available version.
This feature, the ability for end users to subscribe to a Media Gallery to be notified on new files, has been added for v12.