RelayState value given by SP is HTML reencoded
GoogleApps use RelayState to set the back URL.
We receive for example:
{panel} https%3A%2F%2Fwww.google.com%2Fa%2Flinid.org%2FServiceLogin%3Fservice%3Dcl%26passive%3Dtrue%26continue%3Dhttp%253A%252F%252Fwww.google.com%252Fcalendar%252Fhosted%252Flinid.org%252Frender%26followup%3Dhttp%253A%252F%252Fwww.google.com%252Fcalendar%252Fhosted%252Flinid.org%252Frender {panel}
And we return: