Although GPars was bundled into Groovy after 2.1, they apparently removed it in later versions so you will still need to add a dependency.
For example, using the built-in Grape system, you can simply add the following to the top of a Groovy script:
@Grab(group='org.codehaus.gpars', module='gpars', version='1.2.1')
Or, using Gradle:
dependencies {
compile "org.codehaus.gpars:gpars:1.2.1"
}
See GPars website for more information.