The RestApiDoc plugin allows to document your Grails Rest API. Thanks to some Annotations (@), you will be ready to build a full API report (with a playground to perform test request). The plugin is based on jsondoc.This plugin allows you to document methods (description, HTTP path/verb, parameters, response type...) and resources (description, all fields,...). It does a lot of stuff for you (computing path/verb for the method, retrieving field type,...)
Further documentation can be found here.