Since your top-level node <TravelItineraryReadRS>
is not part of that XML namespace, you cannot use the DEFAULT
qualifier. Instead, you have to define a XML namespace prefix, and include that in your XQuery:
;WITH XMLNAMESPACES ('http://webservices.sabre.com/sabreXML/2011/10' AS ns)
SELECT
XmLContent.value('(/TravelItineraryReadRS/ns:TravelItinerary/ns:CustomerInfo/ns:PersonName[1]/ns:GivenName)[1]', 'nvarchar(50)') AS Name
FROM
dbo.RezMonitorXMLdataTest
WHERE
locator = 'GUBXRV'