The 2008 DNS server has a set of root hints (basically the IP addresses of the main root servers).
This means that it doesn't "need" to have any forwarders set. It is entirely up to you. If you set a forwarder to your ISP's DNS server then all requests (other than for domains for which your server is authoritive for) will be forwarded to this server. If their DNS server is down then you won't be able to resolve any names.
I'm not sure what you mean by the second bit, domains don't have default gateways, networks do. All PC's that need to access the Internet (including your DNS server) must have a valid default gateway which would usually be your router.
Cheers
Phil
