IPv4 Properties Settings for servers on an isolated network?

It really doesn't matter what you put as default gateway. All machines are in the same network thus no routing needs to happen so no gateway to another network needs to be contacted.

--edit--

And yes, for DNS, if your server is hosting DNS and the other clients need it then the servers ip should be put into the clients' config as default DNS as you already properly did according to the screenshot.

--edit2-- As others have pointed out in their answers: While putting anything into your default gateway configuration won't break your setup, leaving the default gateway blank is the more elegant way to go.

Leave the default gateway blank. There is no default gateway, because there's no access to any other subnets.