Google requires that the entire email address is passed through to GMail for authentication purposes. If the entire email address is not passed through, you may see a message reading:
Sorry, you've reached a login page for a domain that isn't using Google Apps. Please check the web address and try again."
This issue can be caused when the "@gmail.com" domain is not in the user name field. Double check that your username in the GMail element is set to "[username]@gmail.com" by clicking the pencil icon in the GMail portal element toolbar to bring up the configuration window.