Hi Mirna, you've probably already done this, but I believe the simplest way to do this is to search "guru" in the slack global search. This will return all references to Guru, including links. The other search that you'd need to do is search all of the messages from the Guru app. That should capture the guru app posts of cards directly into channels.
Of course that's a super manual way to do it and may not capture every instance, but I just tested and I believe it returned every card that's been posted in a public channel since we adopted Guru. The much more robust way would be to scrape all public channels using the Slack API.