Hi Arnab,
The LocationCriterionService, which is used in the look up location example, currently only returns limited information, such as only those related to the input. It does not, unfortunately, have the ability to return more advanced searches (retrieve all supported Country type location criteria) such as the one you are looking for.
You may, however, refer to our geo targeting spreadsheet and filter using the Target Type = Country in order to retrieve all supported location criteria of the said type along with their details.
Thanks and regards,
Peter
Google Ads API Team