Hi,
It entierly depend what is the gateway and the typology of our network!
1- Is you ISP provider modem is your gateway? What is it ip address ?
2- Or did you pretend made your Netserver as the gateway?
For the 1st point: If you choose to let the modem as gateway, put the modem ip address as gateway when configuring NS.
The NS is only a client, like any machines on the Network...
For the 2nd point: If you have more than one NIC in your NS instance, you can configure it as gateway (and treat your ISP modem already as internet/untrusted) .
In this scenario, you have to configure the Red Nic as a client of the modem ( probably in DHCP client, let talk of eth0 ), and you have to configure your Green NIC ( the second NIC, let talk about eth1) with an other fixed ip address, and put the same ip address as gateway.
It will be the gateway for all your network clients.
So, for you, what is a more common IP address? I ask, because on the screenshot above.. it's the eth1 configuration.. It mean the second NIC in the NS appliance... It mean the Second scenario...