以下の方法で取得できます。
package sample
import grails.transaction.Transactional
import org.codehaus.groovy.grails.web.servlet.mvc.GrailsWebRequest
import org.codehaus.groovy.grails.web.util.WebUtils
@Transactional
class SampleService {
def serviceMethod() {
GrailsWebRequest webUtils = WebUtils.retrieveGrailsWebRequest()
def request = webUtils.getCurrentRequest()
def response = webUtils.getCurrentResponse()
def session = webUtils.getSession()
println request.aaa
}
}