If some of your client machines are out of reach from the DDP network, you might want to use a gateway which will forward the packets to the right destination.
You can configure one and unique gateway on the DDP in the web GUI "Gateway" page.
Once you enter the gateway IP address in that page, you must make sure that it is reachable from one of the DDP port.
Hence the gateway IP address must be in one for the DDP ports subnets.
Another reason why you would want to specify a gateway on the DDP is in case that you want to connect it to the internet for service connection and you cannot use any DHCP. You might also want to specify a gateway when configuring emails notifications if your SMTP server cannot be directly reached from the DDP.