$core_config[‘smtp’][‘relm’] = ‘gmail.com’; // yes, not realm, its relm
$core_config[‘smtp’][‘user’] = ‘yourusername@gmail.com’;
$core_config[‘smtp’][‘pass’] = 'yourpassword;
$core_config[‘smtp’][‘host’] = ‘smtp.gmail.com’;
$core_config[‘smtp’][‘port’] = ‘587’; (or 465 try both and check which one works out)
The idea is that you should have already setup an mail server (exim, postfix, qmail, sendmail) which in return delivers your email to gmail.
Most linux servers are compiled with sendmail by default, so I suppose that your server is already setup with a mail server.
Do you know how to set up and config a mail server? if Yes have you test it? Can you send and receive emails?
If you don’t want to hassle with setting up a mail server, you should directly go to a paid service like sendgrid or mandrill. They will do the dirty job for you.