I am using ExecuteMultipleResponse method to insert 10 account records at a time using SSIS.
List _Accounts = new List();
//
The guid of a created record should be stored in the OrganizationResponse
which can be found inside the ExecuteMultipleResponseItem
Try the following to get the guid as a string:
string id = response.Response.Results["id"].ToString()
If it works as expected you should also be able to instantiate a guid, if needed:
Guid guid = new Guid(id);