I use Gmail's SMTP server for outgoing mail from Hastymail2. Here are the relevant config file settings:

smtp_server = smtp.gmail.com
smtp_port = 587
smtp_tls = false
smtp_starttls = true
smtp_authentication_type = user

With the smtp_authentication_type set to "user" a few new settings become available on the options page in the compose section. One is the "SMTP authentication" setting which should be set to plain. The other two are the authentication username and password which should be set to your gmail login credentials.

I am not using Gmail's IMAP server directly, rather I use fetchmail to pop messages from Gmail to my local IMAP account.

Jason