I'm working on an Email Campaign web application that is developed using Codeigniter

I'm working on an Email Campaign web application that is developed using Codeigniter Framework. I'm sending emails to multiple users which is a slow process. Any there any way to integrate GMAIL API to send an email campaign instead of GMAIL SMTP? I searched on the internet and just found the code using the SMTP server only. I want to use Gmail API instead of SMTP. It will be appreciated if you guys can help me with it.
Here is the sample of GMAIL SMTP

    $config['useragent']    = 'CodeIgniter';
    $config['protocol']    = 'smtp';
    $config['smtp_host']    = 'ssl://smtp.googlemail.com';
    $config['smtp_user']    = '[email protected]'; // Your gmail id
    $config['smtp_pass']    = 'hello'; // Your gmail Password
    $config['smtp_port']    = 465;
    $config['wordwrap']    = TRUE;
    $config['wrapchars']    = 76;
    $config['mailtype']    = 'html';
    $config['charset']      = 'iso-8859-1';
    $config['validate']    = FALSE;
    $config['priority']    = 3;
    $config['newline']      = "\r\n";
    $config['crlf']        = "\r\n";

    $this->load->library('email');
    $this->email->initialize($config);

    $this->email->from($from_email, $subject);
    $this->email->to($to_email);

    $this->email->subject($subject);
    $this->email->message($message);