I guess you are talking about GateIn www.gatein.org not eXo Portal ?
To build a JSF portlet you will need as JSF Portlet Bridge, such as the JBoss Portlet Bridge.
JBoss Portlet Bridge also allow you to add Seam and RichFaces in the mix but has no hard dependency on those, those are not required at all.
If you have further questions about GateIn please use the GateIn forum: http://www.jboss.org/index.html?module=bb&op=viewforum&f=335
If you have further questions about JBoss Portlet Bridge please use the JBoss Portlet Bridge forum: http://www.jboss.org/index.html?module=bb&op=viewforum&f=273