Groovyのちょっとしたこと「gebでhtmlUnitのオプションを設定する方法」 ref: http://qiita.com/aya_eiya/items/eea2201724cc3556d098

GebConfig.groovy

import geb.driver.DriverRegistry
import geb.driver.NameBasedDriverFactory

driver = (Settings.gebDriver)?Settings.gebDriver:'firefox'

if(driver=='htmlunit'){
  driver = {
    def htmlunitDriver = new NameBasedDriverFactory(classLoader,'htmlunit').getDriver()
    htmlunitDriver.webClient.options.setJavaScriptEnabled(true)
    htmlunitDriver.webClient.options.setThrowExceptionOnScriptError(false)
    htmlunitDriver.webClient.options.setUseInsecureSSL(true)
    htmlunitDriver
  }
}