GateIn is a merge of JBoss Portal and eXo Portal (those 2 projects will end). For everything that can have a spec we used those (Portlet 2.0, JCR for example), also we use common 'standards' such as openSocial.
Some components are from (or derivated from) JBoss Portal, some parts are from eXo Portal. The doc will consolidate the 2, and hopefully we will be able to write tutorials by the GA release. In the meantime if you get to learn how to do things on your own (and the initial documentation that we will provide next week) and if you want to write some articles or doc, we would gladly accept those :)
Thanks for your interest
Eagerly waiting to contribute.