Eclipse and Gradle is used by the OpenCms core developers. And the gradle projects with the OpenCms source are available at GitHub. Moreover, all OpenCms releases are available at MVNrepository. So if you write code depending on the OpenCms source, it's the easiest and most convenient way to stay with Gradle. And Eclipse? Well, it provides a good environment for Java programming and Gradle is integrated easily via a plugin.
You can download the latest Eclipse at www.eclipse.org. For OpenCms development the "Eclipse IDE for Java Developers" is most suitable. If you have already installed another version of Eclipse, you may use it as well.
Once Eclipse is installed, you can start it and add the Gradle plugin. Therefore:
That's it! You have setup your OpenCms development environment. Read here how to start developing dependent on your development scenario.