I am creating this post on behalf of a customer who raised this issue.
Fields Case object::Owner ID Email and Case object::Owner ID Name are not visibile in either JO query builder or Rules Engine for NXT or NXT+SFDC edition customers.
However this is not the case for pure SFDC edition of Gainsight.
More context : Owner field in Case object is looking up to 2 objects (kind of WhatId type field in Gainsight). The lookup tree for these type of fields is not supported in NXT or NXT+SFDC editions.
I request you to kindly implement a enhancement to make these fields as visible as these fields are visible in UI in report builder and not in Rules/JO- Query builder.