Google Hangouts does not support XMPP federation, meaning the ability to connect people on Google Hangouts to people that are not on Google Hangouts.
Ironically, in May 2013 at Google I/O it was Larry Page that was expressing his dissatisfaction with the state of the industry for the lack of interoperability with messaging, lambasting Microsoft. But then just after Microsoft announced their support of XMPP in Outlook.com, Google announces Hangouts.
I guess that was provided as an excuse for dropping XMPP support. Well, welcome to the club Google, now you're part of the problem.