The default gateway defines the address of the router for your network. The default gateway for each client would point at the router interface for that subnet.
For instance, if you defined a subnet 192.168.1.0 and the IP address for the router interface on that subnet were 192.168.1.254, then all the clients in that subnet would have their default gateway set for 192.168.1.254.
You can have a single DHCP server with separate scopes for each subnet. Set the IP Helper in the router to point at this DHCP server.
Are you using a Windows 2000 server as the router? If so, then you would need to have one address from each subnet bound to the NIC in that server. Use the TCP/IP Properties | Advanced option to do this.
You also need to enabled Routing and Remote Access Services (RRAS) in a Network Router configuration. The console for this is under Programs | Administrative Tools.
The Windows 2000 server acting as a router would have no default gateway unless you want it to route to another network such as the Internet. If so, then define a default gateway in RRAS that points at the Internet router.
This was first published in March 2001