If you want to read a file on Google Cloud Storage from an App Engine application, you need to use the Google Cloud Storage Java Client Library, or you can read it using Blobstore API after you get the blobstore key using the function createGsBlobKey.
Using Google Cloud Storage Java Client Library in order to read/write files is fairly simple. Check out this page for more info: https://developers.google.com/appengine/docs/java/googlecloudstorageclient/getstarted