homebrew-grails-1.3.8
Currently Grails Version 1.3.8 is not easily obtainable through homebrew.
Step 1) Checkout and copy 'grails.rb' to '/usr/local/Library/Formula/'
Step 2) $ brew unlink grails $ brew install grails
Step 3) Rejoice!
*Note: You only need to do this once. Homebrew lets you switch between already installed versions easily.
Example:
$ brew swicth grails 1.3.8 //Switches you to 1.3.8
$ brew switch grails 2.2.2 //Switches you to 2.2.2