I'm also looking some documentation on how to do this.
It's possible integrate gatein with eXo WCM, however, eXo team's is working for compatibilty of your product with gatein 3.1.
A new version of eXo WCM, called Site Publisher is available for JBoss Enterprise Portal Platform.
I think it works together with standard GateIn as well as an extension package. Probably you could try it.