This is most likely related to this bug which was fixed last week https://jira.jboss.org/jira/browse/GTNPORTAL-1118
If you are running the Tomcat version of GateIn you will need to edit conf/server.xml and add emptySessionPath="true" to the connector (http://fisheye.jboss.org/browse/gatein/portal/trunk/server/tomcat/patch/src/main/tomcat/conf/server.xml?r1=376&r2=2751)
If you are running the JBoss version of GateIn you will need to edit the server/default/deploy/jbossweb.sar/context.xml to include <SessionCookie path="/"/>
The next update of GateIn will already contain these fixes. Let me know if this fixes your problem or not.
Thank you, Matt.
It fixes my problem.