Ah, I’ve been in the same situation before—this is a really common confusion when using auth.authenticate_user() in Colab. The key thing is that auth.authenticate_user() only authenticates you for Google Cloud services, like BigQuery or Drive if you’re using Colab in “service mode,” but it does not automatically give access to your personal Google account for all APIs. When you call google.auth.default(), it grabs the application default credentials, which is why you’re seeing "default" instead of your Gmail and why you get the 403 insufficientPermissions error. Basically, the Calendar API needs OAuth credentials tied to your personal Google account, not the default Colab service credentials. Since Colab doesn’t have a regular browser for flow.run_local_server(), the usual workaround is to use InstalledAppFlow with flow.run_console() instead. That way, Colab will print a URL you can open manually, log into your personal account, and then paste the code back into Colab. That approach actually gives you proper credentials linked to your Gmail and allows you to access your personal calendar.