How to search for all files that have not been viewed since 01 July 2015.
You can select Drive audit – Apply custom filter and select search parameter – Updated before or equal to July 1, 2015
This will display only the files across your domain that last been updated before July 2015
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 or the new owner becomes the 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.