@Betamaxtest is a tool, written in Groovy, for mocking external HTTP resources such as web services and REST APIs in your tests. First time you contact a third party REST api, it saves the response and the next test run will replay the previously saved response. It works with Spock, thus is a perfect tool for a Grails 3 app. Moreover, It seems a like a perfect tool for some of my company tests which relay in third party calls.
They link to a github repo with a Grails example; although the link is currently broken.