recommit the same changes, will be working on test cases

CHANGELOG.md

@@ -64,7 +64,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Add task completion count in search backpressure stats API ([#10028](https://github.com/opensearch-project/OpenSearch/pull/10028/))
 - Performance improvement for Datetime field caching ([#4558](https://github.com/opensearch-project/OpenSearch/issues/4558))
 - Deprecate CamelCase `PathHierarchy` tokenizer name in favor to lowercase `path_hierarchy` ([#10894](https://github.com/opensearch-project/OpenSearch/pull/10894))
-
+- Added support for Google Application Default Credentials in repository-gcs ([#8394](https://github.com/opensearch-project/OpenSearch/pull/8394))
 
 ### Deprecated
 
@@ -147,4 +147,4 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 ### Security
 
 [Unreleased 3.0]: https://github.com/opensearch-project/OpenSearch/compare/2.x...HEAD
-[Unreleased 2.x]: https://github.com/opensearch-project/OpenSearch/compare/2.12...2.x
+[Unreleased 2.x]: https://github.com/opensearch-project/OpenSearch/compare/2.12...2.x

plugins/repository-gcs/src/main/java/org/opensearch/repositories/gcs/GoogleCloudStorageService.java

@@ -36,6 +36,7 @@
 import com.google.api.client.http.HttpRequestInitializer;
 import com.google.api.client.http.HttpTransport;
 import com.google.api.client.http.javanet.NetHttpTransport;
+import com.google.auth.oauth2.GoogleCredentials;
 import com.google.auth.oauth2.ServiceAccountCredentials;
 import com.google.cloud.ServiceOptions;
 import com.google.cloud.http.HttpTransportOptions;
@@ -213,10 +214,13 @@ StorageOptions createStorageOptions(
             storageOptionsBuilder.setProjectId(clientSettings.getProjectId());
         }
         if (clientSettings.getCredential() == null) {
-            logger.warn(
-                "\"Application Default Credentials\" are not supported out of the box."
-                    + " Additional file system permissions have to be granted to the plugin."
-            );
+            logger.info("\"Application Default Credentials\" will be in use");
+            try {
+                final GoogleCredentials credentials = SocketAccess.doPrivilegedIOException(() -> GoogleCredentials.getApplicationDefault());
+                storageOptionsBuilder.setCredentials(credentials);
+            } catch (IOException e) {
+                logger.error("Failed to retrieve \"Application Default Credentials\"");
+            }
         } else {
             ServiceAccountCredentials serviceAccountCredentials = clientSettings.getCredential();
             // override token server URI