Just brew tap jingweno/gaffer and then brew install gaffer. If the formula conflicts with one from mxcl/master or another tap, you can brew install jingweno/gaffer/gaffer.
You can also install via URL:
brew install https://raw.github.com/jingweno/homebrew-gaffer/master/gaffer.rb
brew help, man brew, or the Homebrew wiki.