If you check the response of request (Developer Tools >> Network Tab) you can see the error:
SMTP connect() failed. https:\/\/github.com\/PHPMailer\/PHPMailer\/wiki\/Troubleshooting
Basically, the PHPMailer couldn’t connect to the gmail server and this can be caused by a lot of reasons that you will need verify one by one. Normally this is a problem related to the server where your website is hosted. So maybe the support of your hosting provider can help you with that.