The short answer:
- Open Eclipse
- Select the menu: Help->Install new software…
- Click the Add… button
- Enter http://dist.springsource.org/snapshot/GRECLIPSE/e4.5/ for the name and URL
- Select groovy packages and install
The longer answer is that that since Pivotal Software has dropped funding for Groovy there is a bit of a transition period and things aren’t as easy to find as they used to be. I also no longer find using STS as beneficial since a quick install to get running was the goal and if I have to install Groovy support separately moving forward with STS too then why not use the standard Eclipse app?