IntelliJ has posted the recording of the Webinar. If you use Grails and IntelliJ, watch this video.
You will see that you can develop Grails 3 apps with IntelliJ IDEA Community IDEA; build, run, debug. It is entirely okay and productive. Grails 3 using Gradle is a big part of it.
However, the Ultimate edition shows other capabilities. GSP editor, wizard for creating artefacts, a visual representation of the relationship between domain classes...
Moreover, learn how to attach a remote debugger to a Grails 3 app with IntelliJ.