Using Gmail Gopher's Bulk Purge Emails feature requires a search results set, output as a CSV file, from the Email Log Search tool in the G Suite Admin Console.
Note: It is important to be aware of the size limitations of Email Log Search outputs. We believe that in most school use-cases (<5000 recipients on the same message), these limitations should not be a problem, however in very large scale send scenarios, it is possible a more robust, scripted approach could be necessary. The Amplified IT Technical Services team has alternatives available in these scenarios. Alternatively, you can upvote the addition of the "Bulk search / delete message by ID for all domain users" feature on our roadmap.
Identifying messages using Email Log Search
Any given email(s) you want to target for deletion, whether spam, phishing attack, or inappropriate / accidental send, will have some combination of details that makes it queryable with the Email Log Search tool (shown below). Some additional, useful fields for narrowing search include Message ID, Sender IP, and Recipient IP.
These additional message details can be revealed by selecting Show original within the Gmail message interface.
Exporting Email Log Search Results
Once you've positively identified the offending messages via Email Log Search query, export them in CSV format as shown below.