I write test suites in Ruby using Rspec and Selenium. This does not mean that I must use Ruby to write automated tests for code written in Ruby. There’s no need to choose your automation tool based on the fact that your application is written in Ruby. However, if you are looking for automation tools/frameworks for automating tests in Ruby then Rspec with Selenium is a popular choice that has plenty of online documentation.
Its a broad question but perhaps this provides a little insight for you.