Short, because the application has exactly one user and collects nothing.
This application is used by one person: the owner of the Google account that issued its OAuth credentials, who is also the owner of this repository. It is not offered to anyone else and has no mechanism for anyone else to use it.
Only data belonging to that same account, and only through Google's official APIs, under the scopes listed on the home page: Gmail, Calendar, Drive and Forms.
OAuth tokens are written to a local file on the owner's machine so the integration does not have to re-authenticate on every run. Any other data is held only for as long as a given command is running. Nothing is retained on any server, because there is no server.
This site is static and served by GitHub Pages. It sets no cookies and runs no scripts. GitHub may log requests as part of operating its service; see GitHub's own privacy statement for that.
The owner can revoke this application's access at any time from their Google Account security settings, which immediately invalidates its tokens.
Via an issue on the source repository.