Se você instalou o Ruby com Homebrew, tente executar brew unlink ruby; brew link ruby
:
$ which sass
$ brew unlink ruby; brew link ruby
Unlinking /usr/local/Cellar/ruby/2.0.0-p0... 20 links removed
Linking /usr/local/Cellar/ruby/2.0.0-p0... 31 symlinks created
$ which sass
/usr/local/bin/sass
Ou adicione /usr/local/opt/ruby/bin
ao caminho.
$ brew --prefix ruby
/usr/local/opt/ruby
$ stat -f%Y /usr/local/opt/ruby
../Cellar/ruby/2.0.0-p0
$ export PATH=/usr/local/opt/ruby/bin:$PATH
$ which sass
/usr/local/opt/ruby/bin/sass
sudo /usr/bin/gem install sass
deve instalar um binário de sass para /usr/bin/sass
.