How to search for all files that have not been viewed since for example, 01 July 2015.
To find the files across your domain that have not been viewed since 1-July-2015
You can select Drive audit – Apply custom filter – Updated before or equal to July 1, 2015
This will display only the files across your domain that last been updated before July 2015
How to delete all files that are shown in the search across all users.
You would need to take ownership of the files using GAT+
To do this you would need to use GAT Unlock
You can request to become the owner of those files – by sending the request to your security officer
Once you become owner you can delete the files.
The files will be accessible on your Google drive.
Here is a link
you can follow for GAT Unlock feature if you need.