Codenarc is a static code analysis tool for Groovy. It's been around for quite a while but one feature it lacked was the ability to see the rule violations right inside of IntelliJ.
Luckily one of my co-workers saw that Daniel Demus recently updated the IntelliJ plugin that was originally created by Cédric Champeau.
One caveat is the plugin currently does not automatically import your
codenarc.groovy|xml rule files - you have to enable rules manually in the inspection settings dialog. Though there is a feature request open to do just that.