You can start a Grails app (one that has an Eclipse Grails nature, i.e. it was created in STS or was converted via Configure | Convert to Grails Project) in STS in debug mode using Run | Debug As | Grails Command (run-app)
You can also attach to any app that you start from the commandline via "grails-debug run-app" by creating a Debug Configuration. Go to Run | Debug Configurations and select Remote Java Application. Click the 'New' button or right-click the Remote Java Application node and select New. Select the project that you're debugging and give the configuration a meaningful name (I usually call them "attach "). Change the port from 8000 to 5005, and I always check the "Allow termination of remote VM" checkbox so I can kill the app from the IDE, but that's optional. Click Debug and it'll attach. Note that this will work for any Grails project that's in your workspace, not just ones that have a Grails nature.
