setting default gateway question

i have a new network switch and i'm using it on my own little LAN.
this has no internet connectivity.
i have a few pc's and i set static ip addresses to them all...
192.168.0.1 for pc-1
192.168.0.2 for pc-2
and so on,
each pc the subnet mask is 255.255.255.0

on my new network switch, i set it's ip address to 192.168.0.100.
but it wants some number for gateway, i cannot leave it blank.
what is the proper ip address to put in there?
is it based on the subnet mask?

it is not a router.
and i am not using a router.
it's a web smart dlink switch that does link aggregate among other things.
the factory setting ip address is 10.90.90.90. that's where you go to in a web browser to access the configuration of the switch for the first time.
I'm using a handful of windows 7 computers connected to this switch and that's it, with each pc having a static ip address.
i'd like the switch on the same 192.168.x.x.
and because i named my computers pc1, pc2, and so on i'd like pc1 to be 192.168.0.1 and pc2 to be 192.168.0.2.
that said, if i arbitrarily choose 192.168.0.200 to be the address of the network switch then what should i set the gateway to be in that switch?
i can't leave it blank like i can in windows 7 setting up a static ip address.