DHCPv6 is defined in RFC 3315.
As such there is no default gateway option in DHCPv6, so we can not assign some specific option for it, to get from DHCP Server.
Actually the default gateway option should come through the Router advertisements (RA)
In windows 7, if we enable DHCP mode, it will accept , the server IP address from DHCPv6 Server and default gateway from RA.
In Fedora core 16, we have to enable mode "Dynamic+Auto" mode to get this behavior.
We have one draft coming up for DHCPv6 route option (http://datatracker.ietf.org/doc/draft-ietf-mif-dhcpv6-route-option/) this will expire in Feb 2013.
Once it comes to RFC status , we will have this option to enable on DHCPv6 servers.
As of now, isc- dhcp server support this option , please see below link for more information.