When accessing a page that requires privileges, a Sign in page is shown and it can be customized by using an extension. To do so, it would be enough to copy the file located at:

gatein.ear/portal.war/login/jsp/login.jsp

and include it in a portal extension such as myExtension.ear/myWar.war/login/jsp/login.jsp 

All the logic must be carefully kept in the login page so that the portal will keep working as it should.

To modify the modal window which pops up when the user decides to sign-in, the extension would have a modified copy of:

gatein.ear/portal.war/groovy/portal/webui/UILoginForm.gtmpl