Basic Auth on Gmail via IMAP and SMTP are not supported by Google and require XOAUTH. Notification channels do not support Gmail and Microsoft 365 for this reason (see at the bottom: Accounts — Zammad documentation )
During Getting started when asked to provide a notification channel configuration there’s actually a “Skip” button allowing you to continue without configuring for the time being.
Zammad tells you what the issue is. Install any dependency of sendmail to solve this. E.g. Postfix.