you can add/update default portal navigation at files located at gatein.ear/02portal.war/WEB-INF/conf/portal/portal/classic.
See detail of page configuration at http://docs.jboss.org/gatein/portal/3.2.0-M01/reference-guide/en-US/html_single/#sect-Reference_Guide-Portal_Navigation_Configuration-Portal_Navigation (pages.xml file) where portlet-ref element represents the name of portlet at your portlet.xml and application-ref element should contain the context name of WAR archive with your portlet.
You should also keep in mind that changes at these files take effect only at the first portal initialization.
Another way to define portal, navigation and pages with portets is to use extension mechanism. You can see some details about simple extension at this doc: http://community.jboss.org/wiki/CreateANewPortalUsingExtension
Feel free to ask for details if there is anything unclear.
Michal Vanco, Portal QA