From the ipv6 mailing list:
Can anyone tell how will a DHCPv6 subscriber learn about the default gateway it has to contact for the subscriber traffic?
A DHCPv6 subscriber is supposed to learn about default gateway from Router Advertisement.
You need to read RFC4861 to understand the relevant details of RA.
Also, can you point me to the document where it is mentioned that a DHCPv6 subscriber has to use RS-RA for resolving gateway address?
Section 7.2.2 of RFC6434 explains this (but the wording could be clearer, perhaps). The basic point is that RFC4861 is effectively mandatory and DHCPv6 is optional.