If you are setting up a static IP address for a computer and need the gateway, finding the gateway address is simple, if you have a computer already setup on the network.

Below is a command which you can run on the configured computer to find the IP address of your gateway.

route -n

You should see at least one entry. If you see several, pick the entry under Gateway that is not 0.0.0.0

This number should be the IP address for your gateway.

In a home network, this is likely the same address as your router.