You need to configure a Grails launch configuration and run it in debug mode.
1. Go to Run -> Debug Configurations...
2. In the tree on the left double click on 'Grails'
It will create a new Grails launch configuration.3. In the form on the right:
a. Give the launch configuration a name4. Apply the configuration.
b. Verify the the selected project is your Grails project
c. In the Grails Command section type 'run-app'
5. Press 'Debug'
This will run your grails application in debug mode.
I hope it helps,