diff --git a/pydantic_settings/sources/providers/gcp.py b/pydantic_settings/sources/providers/gcp.py
index 0885eb84..0ec2cb20 100644
--- a/pydantic_settings/sources/providers/gcp.py
+++ b/pydantic_settings/sources/providers/gcp.py
@@ -40,6 +40,15 @@ def import_gcp_secret_manager() -> None:
         ) from e
 
 
+def _is_not_found_error(exc: Exception) -> bool:
+    try:
+        from google.api_core.exceptions import NotFound
+
+        return isinstance(exc, NotFound)
+    except ImportError:
+        return False
+
+
 class GoogleSecretManagerMapping(Mapping[str, str | None]):
     _loaded_secrets: dict[str, str | None]
     _secret_client: SecretManagerServiceClient
@@ -107,12 +116,26 @@ def _get_secret_value(self, gcp_secret_name: str, version: str = 'latest') -> st
         except Exception:
             return None
 
+    def _get_secret_value_or_raise(self, gcp_secret_name: str) -> str | None:
+        try:
+            return self._secret_client.access_secret_version(
+                name=self._secret_version_path(gcp_secret_name)
+            ).payload.data.decode('UTF-8')
+        except Exception as e:
+            if _is_not_found_error(e):
+                raise KeyError(gcp_secret_name) from e
+            return None
+
     def __getitem__(self, key: str) -> str | None:
         if key in self._loaded_secrets:
             return self._loaded_secrets[key]
 
+        if self._case_sensitive:
+            self._loaded_secrets[key] = self._get_secret_value_or_raise(key)
+            return self._loaded_secrets[key]
+
         gcp_secret_name = self._secret_name_map.get(key)
-        if gcp_secret_name is None and not self._case_sensitive:
+        if gcp_secret_name is None:
             gcp_secret_name = self._secret_name_map.get(key.lower())
 
         if gcp_secret_name:
@@ -204,9 +227,12 @@ def get_field_value(self, field: FieldInfo, field_name: str) -> tuple[Any, str,
         # of the same secret name to be retrieved independently and cached in the GoogleSecretManagerMapping
         if secret_version and isinstance(self.env_vars, GoogleSecretManagerMapping):
             for field_key, env_name, value_is_complex in self._extract_field_info(field, field_name):
-                gcp_secret_name = self.env_vars._secret_name_map.get(env_name)
-                if gcp_secret_name is None and not self.case_sensitive:
-                    gcp_secret_name = self.env_vars._secret_name_map.get(env_name.lower())
+                if self.case_sensitive:
+                    gcp_secret_name: str | None = env_name
+                else:
+                    gcp_secret_name = self.env_vars._secret_name_map.get(env_name)
+                    if gcp_secret_name is None:
+                        gcp_secret_name = self.env_vars._secret_name_map.get(env_name.lower())
 
                 if gcp_secret_name:
                     env_val = self.env_vars._get_secret_value(gcp_secret_name, secret_version)
diff --git a/tests/test_source_gcp_secret_manager.py b/tests/test_source_gcp_secret_manager.py
index e7cabbc6..3aa96b46 100644
--- a/tests/test_source_gcp_secret_manager.py
+++ b/tests/test_source_gcp_secret_manager.py
@@ -16,6 +16,7 @@
 try:
     gcp_secret_manager = True
     import_gcp_secret_manager()
+    from google.api_core.exceptions import NotFound, PermissionDenied
     from google.cloud.secretmanager import SecretManagerServiceClient
 except ImportError:
     gcp_secret_manager = False
@@ -76,7 +77,7 @@ def mock_access_secret_version(name: str):
                 resp = mocker.Mock()
                 resp.payload.data.decode.return_value = secret_values[name]
                 return resp
-            raise Exception(f'Secret not found or access denied: {name}')
+            raise NotFound(f'Secret not found: {name}')
 
         client.access_secret_version = mocker.Mock(side_effect=mock_access_secret_version)
 
@@ -130,11 +131,34 @@ def test_secret_manager_mapping_secret_names(self, secret_manager_mapping):
 
     def test_secret_manager_mapping_getitem_access_error(self, secret_manager_mapping, mocker):
         secret_manager_mapping._secret_client.access_secret_version = mocker.Mock(
-            side_effect=Exception('Access denied')
+            side_effect=PermissionDenied('Access denied')
         )
 
         assert secret_manager_mapping['test-secret'] is None
 
+    def test_case_sensitive_getitem_skips_list_secrets(self, mock_secret_client):
+        """With case_sensitive=True, fetching a secret must not call list_secrets."""
+        mapping = GoogleSecretManagerMapping(mock_secret_client, project_id='test-project', case_sensitive=True)
+
+        assert mapping['test-secret'] == 'test-value'
+        mock_secret_client.list_secrets.assert_not_called()
+
+    def test_case_sensitive_getitem_not_found_raises_keyerror(self, mock_secret_client):
+        """With case_sensitive=True, a missing secret (NotFound) must raise KeyError."""
+        mapping = GoogleSecretManagerMapping(mock_secret_client, project_id='test-project', case_sensitive=True)
+
+        with pytest.raises(KeyError):
+            _ = mapping['nonexistent-secret']
+        mock_secret_client.list_secrets.assert_not_called()
+
+    def test_case_sensitive_getitem_permission_denied_returns_none(self, mock_secret_client, mocker):
+        """With case_sensitive=True, a PermissionDenied error must return None (not raise KeyError)."""
+        mock_secret_client.access_secret_version = mocker.Mock(side_effect=PermissionDenied('Access denied'))
+        mapping = GoogleSecretManagerMapping(mock_secret_client, project_id='test-project', case_sensitive=True)
+
+        assert mapping['test-secret'] is None
+        mock_secret_client.list_secrets.assert_not_called()
+
     def test_secret_manager_mapping_iter(self, secret_manager_mapping):
         assert list(secret_manager_mapping) == ['test-secret']