You should be able to do "grails run-app" after unzipping. I'm still learning grails so pardon some of the unconventional coding :)

UPDATE: Grails has upgraded since the I posted this. So additionally run "grails upgrade" on the downloaded project and you should be good to go.

the demo showcases
 a) Wicket + Grails
 b) Spring dependency injection 

the wicket  code is under the "pages" folder under grails-app (ignore the duplicate pages folder that is empty) 

1) You will have to create the database "test" with testuser/testpassword mentioned in DataSource.groovy
2) after starting up the app you can access the wicket app at http://localhost:8080/GrailsWicketDemo/app/
the password is "testing"
3) If you run into any issues please refer to my blog  at http://grailslog.posterous.com where I posted all the wicket issues I encountered and their resolutions.