A lot of folks new to Groovy quickly ask the question, "What's is the GDK?" or "What's the difference between the GDK and the JDK?"
If you take a look thru the GDK documentation, you will see that the GDK acts like an extension to the Java JDK. New methods are added to the GDK help remove the ceremony and allow Groovy code to be short and concise. Think of it as a big decorator pattern! Slight clarification: the JDK is used to build the GDK and referenced by the GDK but it is NOT part of the GDK and not shipped with the GDK.
Maybe a picture will help. You can see some of the new GDK methods added to the Java classes below.
Hope this helps!