Grails Script (gant) で session を flush する ref: http://qiita.com/yuki-takei/items/3856c9a65c328d3d65b4

Test.groovy

def persistenceInterceptor = appCtx.containsBean('persistenceInterceptor') ? appCtx.persistenceInterceptor : null
persistenceInterceptor?.init()
try {

    // サービスメソッドをcall
    def service = appCtx.getBean("userService")
    service.updateUser()

}
finally {
    persistenceInterceptor?.flush()
    persistenceInterceptor?.destroy()
}

UserService.groovy

@Transactional(propagation=Propagation.REQUIRES_NEW)
def updateUser() {
    User user = User.findById(1L)
    user.doSomething()
    user.save()
}