Creating a serving URL for images stored in GCS is slightly different than for images stored in the Blobstore. Here’s a code sample:
Bucket represents the name of your Google Cloud Storage bucket, and object represents the file name of the image (including extension).
String bucket = "bucket_name";
String object = "file_name_including_extension";
//Get serving url
String gs_blob_key = "/gs/" + bucket + "/" + object;
BlobKey blob_key = BlobstoreServiceFactory.getBlobstoreService().createGsBlobKey(gs_blob_key);
ServingUrlOptions serving_options = ServingUrlOptions.Builder.withBlobKey(blob_key);
String serving_url = ImagesServiceFactory.getImagesService().getServingUrl(serving_options);
System.out.println("Serving URL: " + serving_url);
resp.getWriter().println(serving_url);