GreetingController.groovy

class GreetingController {
    def index = {
        [message: message(code: "greeting.message", args: [params.name])]
    }
}

GreetingControllerSpec.groovy

import grails.plugin.spock.*
import org.springframework.context.*
import org.springframework.context.support.*
import spock.lang.*

class GreetingControllerSpec extends ControllerSpec {

    @Shared def messageSource = new StaticMessageSource()
    @Shared def pirateEnglish = new Locale("en", "BV")

    def setupSpec() {
        messageSource.useCodeAsDefaultMessage = true
        messageSource.addMessage "greeting.message", pirateEnglish, "Ahoy there {0}!"
    }

    def setup() {
        mockMessageTag(controller, messageSource)
    }

    @Unroll
    def "greeting is rendered by index action"() {
        given:
        if (name) controller.params.name = name
        if (locale) controller.request.addPreferredLocale(locale)

        expect:
        controller.index() == [message: message]

        where:
        name  | locale        | message
        null  | null          | "greeting.message"
        "Rob" | null          | "greeting.message"
        "Rob" | pirateEnglish | "Ahoy there Rob!"
    }

    // in reality this would be static imported from a helper class
    static void mockMessageTag(artefact, MessageSource messageSource) {
        artefact.metaClass.message = { attrs ->
            messageSource.getMessage(attrs.code, attrs.args as Object[], delegate.request.locale)
        }
    }
}