The Groovy plugin requires the definition of the 'groovy' dependency like this:

apply plugin: 'groovy'

dependencies {
    groovy group: 'org.codehaus.groovy', name: 'groovy', version: '1.7.6'
}

Gradle already ships with a groovy-all library. I think it would be great if we could simply assign that library so the user doesn't have to define the dependency. If the user wants to assign a different Groovy version he/she can still do that in the dependencies closure. At the moment I am using this workaround:

dependencies {
    groovy fileTree(dir: new File(gradle.gradleHomeDir, 'lib'), includes: ['**/groovy-all-*.jar'])
}