net-misc/gogoc sets a metric of 1 for the 2000::/3 route, which overrides any default router the host might pick up via RAs. This means that a host running gogoc (such as a laptop) on a dual-stack network will have IPv6 connectivity both through the native network connection and via the tunnel, but will favor the tunnel over the native connection!
Sounds like something you may want to request upstream? http://gogonet.gogo6.com/
Sorry I thought I had closed this bug after tomwij's comment! Cleaning up now