Login to this Gitter page to obtain the token you need to use as the password for the IRC connection. Now add a new server from Smuxi's menu: Server -> Add and fill in the following details:
Connect to Gitter by clicking the Connect button from the Smuxi menu bar, select irc.gitter.im and confirm with the Connect button.
Protocol: XMPP Hostname: talk.google.com Port: 5222 Username: firstname.lastname@example.org Password: your password or an Google application password Use Encryption: checked / enabled
2 factor auth: https://support.google.com/accounts/answer/185833?hl=en
plain password auth: https://www.google.com/settings/security/lesssecureapps