diff --git a/core/services/gcs/src/backend.rs b/core/services/gcs/src/backend.rs
index c038fb5de58c..3648e7967bc2 100644
--- a/core/services/gcs/src/backend.rs
+++ b/core/services/gcs/src/backend.rs
@@ -357,6 +357,9 @@ impl Builder for GcsBuilder {
             write_with_content_type: true,
             write_with_content_encoding: true,
             write_with_user_metadata: true,
+            // Honored on single-shot writes (JSON upload + ifGenerationMatch=0).
+            // Multipart XML uploads cannot enforce preconditions and return
+            // Unsupported when if_not_exists is set — see GcsWriter::initiate_part.
             write_with_if_not_exists: true,
 
             // The min multipart size of Gcs is 5 MiB.
diff --git a/core/services/gcs/src/writer.rs b/core/services/gcs/src/writer.rs
index 9e50e3b9e0c5..b594d1836ef3 100644
--- a/core/services/gcs/src/writer.rs
+++ b/core/services/gcs/src/writer.rs
@@ -72,6 +72,21 @@ impl oio::MultipartWrite for GcsWriter {
     }
 
     async fn initiate_part(&self) -> Result<String> {
+        // GCS XML API multipart uploads cannot carry request preconditions
+        // (`ifGenerationMatch` / similar). Single-shot `write_once` honors
+        // `if_not_exists` via the JSON upload API; the multipart path must not
+        // silently overwrite an existing object when the caller asked for a
+        // conditional create.
+        //
+        // ref: https://cloud.google.com/storage/docs/request-preconditions
+        // ref: https://github.com/apache/opendal/issues/8040
+        if self.op.if_not_exists() {
+            return Err(Error::new(
+                ErrorKind::Unsupported,
+                "gcs multipart upload cannot honor if_not_exists; use write() for conditional creates",
+            ));
+        }
+
         let resp = self
             .core
             .gcs_initiate_multipart_upload(&self.ctx, &self.path, &self.op)
diff --git a/core/tests/behavior/async_write.rs b/core/tests/behavior/async_write.rs
index 5a17dc69cf82..4076cee687be 100644
--- a/core/tests/behavior/async_write.rs
+++ b/core/tests/behavior/async_write.rs
@@ -821,14 +821,6 @@ pub async fn test_writer_write_with_if_not_exists(op: Operator) -> Result<()> {
         return Ok(());
     }
 
-    // GCS XML API multipart uploads do not support preconditions, so the multipart
-    // writer path cannot honor if_not_exists. Tracked in
-    // https://github.com/apache/opendal/issues/8040
-    #[cfg(feature = "services-gcs")]
-    if op.info().scheme() == services::GCS_SCHEME {
-        return Ok(());
-    }
-
     let path = TEST_FIXTURE.new_file_path();
     let content = gen_fixed_bytes(cap.write_multi_min_size.unwrap_or(1));
 
@@ -846,6 +838,18 @@ pub async fn test_writer_write_with_if_not_exists(op: Operator) -> Result<()> {
         Ok(())
     }
     .await;
+
+    // GCS XML multipart uploads cannot enforce if_not_exists, so the writer fails
+    // closed with Unsupported instead of ConditionNotMatch / silent overwrite.
+    // Single-shot write_with_if_not_exists still works via the JSON API.
+    //
+    // ref: https://github.com/apache/opendal/issues/8040
+    #[cfg(feature = "services-gcs")]
+    if op.info().scheme() == services::GCS_SCHEME {
+        assert_eq!(res.unwrap_err().kind(), ErrorKind::Unsupported);
+        return Ok(());
+    }
+
     assert_eq!(res.unwrap_err().kind(), ErrorKind::ConditionNotMatch);
 
     Ok(())