From what I've seen on other forums it seems that map updates are not available for 2008 Wings because of the software that runs the system. So guys have tried (with some success) to update the firmware and then installed the latest map updates. Sorry I can't be more specific or helpful, maybe someone who has a NAV GW can chime in.
I read somewhere where Garmin was charging $199.99 for the up date and that soon wouldn't be supporting the Gold Wing NAVI system anymore. Your better off buying an after market GPS and mounting it on your Wing, it's cheaper and updates cost less, and you can do a lot more with them.
For what ever it is worth, here is a copy of a response I received from Garmin when asking about map and system updates. I agree with above, August is almost over. I received this email on August 17th, this year.
Dear Craig Bryant,
Thank you for contacting Garmin International. I will be happy to help you with this. The latest version of mapping for your Navi is v9 (2009). It was released in Aug. 2009 and retails for $149.00. I do know that we will continue to support and update these units thru 2016 per our agreement with Honda. That being said I do not have any dates for the release of new updates. The update can be ordered by following the following link: