Have you tried groovy-wslite
? It says in the documentation for GroovyWS that:
Due to low availability the project is currently dormant, you might consider groovy-wslite as a replacement module
So afaics, this should get close (I'm assuming you're using SOAP):
import wslite.soap.*
def client = new SOAPClient('http://192.168.3.69/provider/myService')
client.authorization = new HTTPBasicAuthorization( 'user', 'pass' )
There are instructions for getting wslite working with Grails here.