If you have shared a document/file with external users and you wish to remove them, click on the ‘Share’ button on the top right of Google Docs page.
Once you have clicked on ‘Share’ a pop up will appear, select ‘Advanced’ from here you can modify the sharing permissions.
From here you can change the sharing permission by clicking on ‘Change…’ and either modify or remove the sharing type.
Finding files shared with a link using GAT
If you have received an email from the GAT+ tool, you can click on the files name to be taken to your drive where the file is currently stored.