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.

To modify the modal window which pops up when the user decides to sign in, the extension contain a modified copy of gatein.ear/portal.war/groovy/portal/webui/UILoginForm.gtmpl