Groovy completed successfully its move to the Git SCM this summer, thanks to Matthew McCullough's help.
For helping everybody get up-to-speed with Git, I've added a page about Git on our wiki.
We now have a "groovy" organization on GitHub: https://github.com/groovy/groovy-core
Our three main branches have been pushed there: master, GROOVY_1_8_X and GROOVY_1_7_X.
The GitHub repository serves as a mirror, and the Groovy despots will be able to regularly (manually for now) push the latest changes to GitHub, and deal with pull requests and such.