You can install Ruby/Git like this:

sudo gem install schacon-git --source=http://gems.github.com

Since there were two major projects that provided this functionality (Grit and Ruby/Git), Tom and I have merged our efforts into a single library to rule them all.
Grit
is where I’m spending my development cycles these days and is the one being actively maintained.

If you are using Ruby/Git, I will take patches if needed, but I will no longer be doing development on it myself. If you are looking for a library to use Git from Ruby, or would like to help develop it with me, please choose Grit. (schacon/grit)