I solved my problem, that was due to my ignorance about these themes.
As the name says, DNS mean Domain Name System, i needed a domain.
So i purchased a domain, then i went on my account of goDaddy.com and set the DNS of the domain registered with another registar, making it point to my instance of Compute Engine.
Now my server XMPP is reachable by others XMPP servers, included gmail.
I found very helpful these links:
http://blog.kenweiner.com/2008/05/ejabberd-on-amazon-ec2-ubuntu-ami.html
http://support.godaddy.com/help/article/4041/managing-domain-names-with-offsite-dns
I hope this is helpful for someone :-)