SMTP (Credentials): Able to log in using GMail username and password
SMTP (App Password): Able to log in using GMail username and app password
Local MTA: Set outgoing email using sendmail and continue the Getting Started Wizard
Actual behavior:
SMTP (Credentials): Kept getting “auth failed” even tho the credentials are correct. Most probably due to OAuth, but I don’t see any way to use OAuth during this stage
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.