Here is where you can find the sources of the latest and greatest NetBeans Gradle plugin:
To use it, download the sources above, open the sources into the IDE (which must be 7.1.1 or above), then you'll have a NetBeans module. Right-click it to run the module into a new instance of NetBeans IDE.
In the Options window's Miscellaneous tab, there's a Gradle subtab for setting the Gradle location. In the New File dialog, in the Other category, you'll find a template named "Empty Gradle file". Make sure to name it "build" and to put it in the root directory of the application (by leaving the Folder field empty, you're specifying it should be created in the root directory).
You'll then be able to expand the build.gradle file:
Double-click a task to run it. When you open the file, it opens in the Groovy editor, if the Groovy editor is installed. When you make changes in the file, the list of tasks, shown above, is automatically recreated.
It's at a really early stage of development and it would be great if developers out there would be interested in fixing its bugs and adding more features to it.