The Page- and Step-Level Alerting feature was introduced as part of the Spring 2012 Release. The related Alerting Web Service API component was part of the Summer 2012 Release.
The Alerting Web Service is supported by Alerting WS 2.0, while the legacy WS XML is not.
The page http://gsr.webservice.gomez.com/AlertManagementService/AlertManagementWS.asmx?op=GetAlertHistory is the legacy version of the Alerting WS XML.
Please use the Alerting WS 2.0 to retrieve the step-level alerting record:
The StartTime and EndTime format is YYYY-MM-DD hh:mm:ss.
For more information, see Alert Management API in the documentation.