Add a router before or instead of the dnslookup router:
gmail_route:
driver = manualroute
transport = gmail_relay
route_list = * smtp.gmail.com
Add a transport:
gmail_relay:
driver = smtp
port = 587
hosts_require_auth = $host_address
hosts_require_tls = $host_address
Add an authenticator (replacing myaccount@gmail.com and mypassword with your own account details):
gmail_login:
driver = plaintext
public_name = LOGIN
hide client_send = : myaccount@gmail.com : mypassword
$host_address is used for hosts_require_auth and hosts_require_tls instead of smtp.gmail.com to avoid occasional 530 5.5.1 Authentication Required errors. These are caused by the changing IP addresses in DNS queries for smtp.gmail.com. $host_address will expand to the particular IP address that was resolved by the gmail_route router.