раз
nodes= tree7.xpath("//*[local-name() = 'outputTree']/*[local-name() = 'command']/*[local-name() = 'pivotTable' and @varName='gender']")

два
ns = {"oms": "http://www.ibm.com/software/analytics/spss/xml/oms"}
nodes= tree7.xpath("/oms:outputTree/oms:command/oms:pivotTable[@varName='gender']", namespaces = ns)