Gradle Api Sources and Doc when writing Gradle Plugins

This works for me in Eclipse:

plugins.withType(EclipsePlugin) {
    plugins.withType(JavaBasePlugin) {
        eclipse {
            classpath {
                file {
                    whenMerged { classpath ->
                        String gradleHome = gradle.getGradleHomeDir()
                            .absolutePath
                            .replace(File.separator, '/')
                        String gradleSourceDirectory = "${gradleHome}/src"
                        classpath.entries.each { entry ->
                            if (entry in org.gradle.plugins.ide.eclipse.model.AbstractLibrary
                                    && entry.library.path.contains('generated-gradle-jars')) {
                                entry.sourcePath =
                                    new org.gradle.plugins.ide.eclipse.model.internal.FileReferenceFactory()
                                        .fromPath(gradleSourceDirectory)
                            }
                        }
                    }
                }
            }
        }
    }
}

Make sure that your Gradle Home contains the source directory. If you use the wrapper this can be done by updating the distributionUrl to an -all version in the wrapper task.

You will also need to stop the Gradle daemons that are running otherwise they will keep their own "home": ./gradlew --stop, then you can go ahead and run the task: ./gradlew eclipse

See GRADLE-2133 (which this answer was adapted from)