getNodeValue return null if I add ns

Namespace is not added in your response. And also, getNodeValue() gets the exact xpath of the response.

So, for this particular line to be executed properly:-

def a = holder.getNodeValue("//ns:table[country='CountryA']//*:city")

Your response should look like this:-

<soap:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
   <soap:Body>
      <CheckCity xmlns="http://some ns here">
         <Result>
            <ns:table>
               <country>CountryA</country>
               <city>CityA</city>
            <ns:/table>
            <ns:table>
               <country>CountryB</country>
               <city>cityB</city>
            <ns:/table>
            <ns:table>
               <country>CountryC</country>
               <city>cityC</city>
            <ns:/table>
         </Result>
      </CheckCity>
   </soap:Body>
</soap:Envelope>