Once I post JSON data to a url in Grails, how can I get access to that data inside of the controller?
Grails automatically parses/unmarshals the JSON and you can access it via request.JSON in your controller. The object returned is of type JSONObject and thus allows map-style access to the properties. You can also directly use this JSONObject for data binding:
def jsonObject = request.JSON
def instance = new YourDomainClass(jsonObject)