Since we released the Groovy course, there have been some changes to the hosting of the Groovy distribution files. The newer versions of Groovy are completely compatible with the course, so you're fine to use any, but you'll need to download them from:
Please note that for Windows users, the latest version may not have a windows installer available. To get started quickly you may wish to check a prior version with a Windows installer. At the time of creating this blog post, 2.4.11 is the latest stable version of Groovy, but if you want the Windows installer you'll need to get 2.4.10.