Re: Spring Security Core 2.0-X: How to customize the login page and controllers?
In the what's new section it's explained how it works:
Also, the plugin includes the grails.plugin.springsecurity.LoginController.groovy and grails.plugin.springsecurity.LogoutController.groovy controllers, and grails-app/views/auth.gsp and grails-app/views/denied.gsp GSPs. If you had no need previously to change these you can delete your files and the plugins' files will be used instead. If you do want to change them, copy each as needed to your application and make the required changes, and yours will be used instead.