The gobby download page somewhat unhelpfully states that “Gobby runs on Mac OS X, however we cannot offer a pre-built package at the moment.”
Someone’s compiled it and throw it into brew’s repository, though.
brew install homebrew/gui/gobby
Install XQuartz (local X server on OSX, as far as I’ve gathered).
Find gobby’s executable on your machine (probably
/usr/local/Cellar/gobby) and open it in XQuartz.