Check to make sure that you are correctly entering your e-mail address and password. To verify this go to gmail.com in your web browser and type in the same credentials as you are using in PocketCloud. Verify that you can login. If you cannot then you should use the "Forgot Password" feature on gmail.com.
If you are sure your Gmail credentials are correct, you may have to sign into Gmail through the browser on your mobile device for authorization. This is due to new security steps Google has put into place for some Gmail accounts.