Network Configuration -> Hosts
Add an entry for the router (gateway).

Network Configuration -> Gateways
Select the Host Table entry created above.
Set the Service to Default
Add the Gateway Entry

Save the configuration to Flash

In order for the unit to rebuild the route table it must be rebooted.

Jetstream 8500 example below shows the Command Line options to create a Host Table entry, create a Gateway Table entry, and setting the Gateway to passive mode (this is only required on the Jetstream or CS9000 to set the entry as static):

JS_8500# add host router 192.168.1.1
JS_8500# add gateway router default
JS_8500# set gateway router passive default
JS_8500# show gateway
hostname service internet address status
router   default                  passive

Note that the DS Family does not use the passive setting

Refer to your User Guide for further details.