This isn't directly obvious, so I'll clarify. Anyone who has used this app - all of your notes are publicly available by following the link above.

Everyone - revoke access to the app and manually delete all the gists it created in your account.

To view anyone's notes, all you have to do is visit https://gist.github.com/kobble-git/87ea625d421177f9e9307c6ce..., click anyone's "View fork", then follow the trail of gist links in the JSON.