Reading directly on the Groovy Client Preface page, I find the Maven coordinates to use the client library. I even find a link directly to a Maven Central search to find the elasticsearch-groovy library.
What I do not find is any instance of the Groovy client newer than 2.1.2 dated February 2016 in Maven Central, whereas the coordinates provided on the "Preface" page list the current version as 5.4.2.
Perhaps something is amiss? Is the Groovy client still being developed/released?