Newest version of HomeBrew, versions no longer supported
brew tap homebrew/versions
then:
brew install homebrew/versions/grails23
brew search grails will show you all the available ones to install.
It seems to be very easy as of right now:
user@MacBook-Pro:~/% brew search grails
grails ✔ [email protected]
user@MacBook-Pro:~/% brew install [email protected]