If you have SSL enabled on the Gmail account, port number is 587. I believe the other is 465, as per the help docs: https://support.google.com/mail/answer/78775?hl=en
If you tried configuring your SMTP server on port 465 (with SSL) and
port 587 (with TLS), but are still having trouble sending mail, try
configuring your SMTP to use port 25 (with SSL).
You should probably also be using a try/catch block - if sendmail is failing with an error, you can print_r or var_dump to see what the exception message contains.
I should admit I haven't ever used sendmail directly; I found the Rmail library a lot easier to use:
<?php
require_once( ROOT . DS . "libs" . DS . "Rmail" . DS . "Rmail.php" );
// Instantiate a new HTML Mime Mail object
$mail = new Rmail();
$mail->setSMTPParams($host = 'smtp.gmail.com', $port = 587, $helo = null, $auth = true, $user = 'gmail address', $pass = 'password');
// Set the From and Reply-To headers
$mail->setFrom( "Proper Name <send from email address>" );
$mail->setReturnPath( "reply email address" );
// Set the Subject
$mail->setSubject( 'Email subject/title' );
$html = 'you email message content';
// Create the HTML e-mail
$mail->setHTML( $html );
// Send the e-mail
$result = $mail->send( array( 'target email address' ) );
?>
I can track down the library somewhere if you'd rather use something like the snippet above.