I think you can do this behaviour in your extension, re-define the login form with particular message as you want
what's '"extension" you talking about? I have no idea about this.
GateIn provided an extension mechanism which be possible to overwrite portal resources. And with extension, you can also overwrite the login.jsp file and do what you want.
You should read http://docs.jboss.com/gatein/portal/3.1.0-FINAL/reference-guide/en-US/html_single/index.html#sect-Reference_Guide-Foundations-Extension_mechanism for more information about gatein extension and sample at http://anonsvn.jboss.org/repos/gatein/portal/tags/3.1.0-GA/examples/extension/