I would recommend Ruby or Python since you have a Java background. Groovy can allow you to lean on your Java habits, and slow your exposure to a more dynamic way of thinking.
I have over six months professional experience in Groovy, and there is a lot to recommend the language. But it is ultimately a compromise, designed to include some of the nice features of Ruby and Python, but with the constraining goal of Java compatability.
Also, the documentation is not nearly as good as Python, also probably not as good as Ruby, and the community support is not nearly what you will have in either Python or Ruby.
The only advantage of using Groovy is that it will allow you to continue to be productive today, as you can essentially write Java whenever you aren't sure what to do. But if your primary goal is learning, choose either Ruby or Python. (Base your decision on which of these languages appeals more to you philosophically.)