Here is an example of defining and using the namespace. You can adapt it to your actual query. Copy and paste this to an SSMS query window and run to see what it does.
DECLARE @x XML =
'<gml:Envelope xmlns:gml="http://somenamespace" srsName="urn:ogc:def:crs:EPSG::27700">
<gml:lowerCorner>0 0</gml:lowerCorner>
<gml:upperCorner>1225000 660000</gml:upperCorner>
</gml:Envelope>'
;WITH xmlnamespaces ('http://somenamespace' AS gml)
SELECT
Envelope.value('./@srsName[1]','varchar(255)') as 'srsName'
FROM
@x.nodes('//gml:Envelope') as x(Envelope)