While the TestRail Ruby client for their V2 API is available on Github, it isn't available as a gem. Ain't nobody want to be copying and pasting API clients into their projects.
And thus, zachpendleton/testrail was born. It's a gem wrapped around the example Gurock client.
Add this line to your application's Gemfile:
gem 'testrail', git: 'git://github.com/zachpendleton/testrail.git'
And then execute:
$ bundle
Read more about Gurock's client over at their repo.