Configuring postfix for virtual host using specific smtp (gmail)

i’ve configured using gmail as for my server for sending out email “globally” like the following tutorial

http://mhawthorne.net/posts/postfix-configuring-gmail-as-relay.html

But my server is hosting multiple domain. So, every domain email will send through above gmail, how can i config each virtual host using its gmail smtp server?

for example
domainA.com will use domainA@gmail.com
domainB.com will use domainB@gmail.com

i found a tutorial in ubuntu, but i’m using centos and webmin+virtualmin

http://blog.otelconsulting.com/2013/03/ubuntu-php-postfix-virtual-hosting-multiple-relay-host-via-gmail/

how to do this in webmin?