Don't display the warning is not prompting anyway.

gcl.py

 #!/usr/bin/python
 # Copyright (c) 2006-2009 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 #
 # Wrapper script around Rietveld's upload.py that groups files into
 # changelists.
 
 import getpass
 import os
@@ skipping 1022 matching lines @@
   # Need to import here to avoid circular dependency.
   import presubmit_support
   root_presubmit = GetCachedFile('PRESUBMIT.py', use_root=True)
   result = presubmit_support.DoPresubmitChecks(change_info=change_info,
                                                committing=committing,
                                                verbose=False,
                                                output_stream=sys.stdout,
                                                input_stream=sys.stdin,
                                                default_presubmit=root_presubmit,
                                                may_prompt=may_prompt)
-  if not result:
+  if not result and may_prompt:
     print "\nPresubmit errors, can't continue (use --no_presubmit to bypass)"
   return result
 
 
 def Changes():
   """Print all the changelists and their files."""
   for cl in GetCLs():
     change_info = ChangeInfo.Load(cl, True, True)
     print "\n--- Changelist " + change_info.name + ":"
     for file in change_info.files:
@@ skipping 103 matching lines @@
     # the files. This allows commands such as 'gcl diff xxx' to work.
     args =["svn", command]
     root = GetRepositoryRoot()
     args.extend([os.path.join(root, x) for x in change_info.FileList()])
     RunShell(args, True)
   return 0
 
 
 if __name__ == "__main__":
   sys.exit(main())