This guide was written using Grails 2.4.4 although it should work the same for Grails 2.3.x and 3.x (almost the same :-P)
Since current official documentation only covers unit testing for controllers and that it took me a while to to figure them out completely, I thought I would leave it here in case it helps someone else.
The reason for writing these tests was to validate some REST API endpoints implemented using Grails Controllers.