brew gem allows you to install any rubygem as a homebrew formula.
It works by generating a stub formula for homebrew, which looks something like this:
class Ronn < Formula
  def initialize(*args)
    @name = "ronn"
    @version = "0.7.3"
    super
  end
  def install
    system "gem", "install", name, "--version", version, "--install-dir", prefix
  end
end
This formula installs and unpacks all the dependencies under the Cellar path. So the package is completely self contained.
brew install brew-gem
brew gem heroku
This is not for installing development libraries, but for standalone binary tools that you want system wide.