This is how to work with a chrombrowser instance: 

 

package org.ragin.test

import spock.lang.*
import geb.Browser
import org.openqa.selenium.chrome.*
import org.openqa.selenium.firefox.FirefoxDriver

class GebSpec extends spock.lang.Specification {
    // run before the first feature method
    def setupSpec() {
        println "These are browser tests"
        System.setProperty("webdriver.chrome.driver", "E:\\Dev\\chromedriver\\chromedriver.exe");
    }

    // run after the last feature method
    def cleanupSpec() {
    }

    // run before every feature method
    def setup() {
    }
    // run after every feature method
    def cleanup() {
    }

    def "G01 - Chrome google Ventacor"() {
        given:
        def browser = new Browser(driver: new ChromeDriver())

        when:
        browser.go "http://www.google.com/"
        browser.$("input", name: "q").value("ventacor")

        then:
        assert browser.title == "Google"

        cleanup:
        browser.quit()

    }

    def "G02 - we are able to fetch the legal information about greach"() {
        given:
        def browser = new Browser(driver: new ChromeDriver())

        when:
        browser.go 'http://www.xxx.org/test/test.html'

        then:
        browser.$("h1").text() == "This is the first h1 element"
        browser.$("h3").text() == "This is the first h3 element"

        cleanup:
        browser.quit()
    }
}