GateIn currently does not have this feature, it has been asked about a few times on the forum and a task has been created for it https://jira.jboss.org/browse/GTNPORTAL-870. It is something that needs to be looked into.
GateIn does have the concept of extending a portal (see the sample extension). This will allow for things like adding pages and populating them with portlets, and can be setup in a portlet's war file with xml files. But, its a bit more complex than the way JBoss Portal did things and is _only_ deployable when the portal is first initiated (ie if the server has been started once before, its too late).