I have a Git repository on GitHub that has already been established. Now, I want to mirror that same repository onto Google Code. Either that, or I want Google Code's Source tab to either disappear or point to GitHub. Since that doesn't seem possible, how do I sync the two repositories? All the other answers don't cover if the first Git repo is already established.
Assuming you already set up a remote for Github named origin (as you will if you followed Github's instructions), you can add a second URL to it for Google Code:
git remote set-url --add origin https://project.googlecode.com/git
Now you'll automatically push to both Github and Google Code at once when you git push to that remote. That's probably the easiest way to keep them synced.