Mantis can notify bug status changes via email. It does support Gmail as the email provider as well, however, it is not set up out of the box and needs configuration. It is easy to set up for Gmail by modifying config_inc.php file and including the SMTP specs for the Gmail account.
Gmail support only TLS or SSL so set the port accordingly.
Modify config_inc.php by adding the following in the config file:
$g_allow_signup = ON; //allows the users to sign up for a new account
$g_enable_email_notification = ON; //enables the email messages
$g_phpMailer_method = PHPMAILER_METHOD_SMTP;
$g_smtp_host = 'smtp.gmail.com';
$g_smtp_connection_mode = 'tls';
$g_smtp_port = 587;
$g_smtp_username = 'email@example.com'; //replace it with your gmail address
$g_smtp_password = '*********'; //replace it with your gmail password
$g_administrator_email = 'firstname.lastname@example.org'; //this will be your administrator email address
Login to the account, change bug status and verify that email has been recieved.