How to configure postfix as smtp relay for Gmail on Archlinux

It can be very convenient, for those who use services such as zoneminder, rtorrent, etc., to have a Gmail Relay to send mail from your Gmail account. In this guide we will see how to configure postfix to send email with our account [email protected] The guide is based on ArchLinux, but apart the installation of the packages, the configuration is the same also on other distributions such as Debian or Centos.