So I'm setting up a windows deployment server on my network.
Don't forget that you need to open the following ports on the WDS machine:
(Port 67 is still required if your DHCP server is on a different physical machine).
Additionally, I'm not sure if you need to set Option 67 in a simple environment, so for testing sake, I suggest you remove it and put it back if later if you need a more complex environment. I just had a quick look at our DHCP server that we use with PXE booting all the time, and I don't see it set anywhere, only Option 66.
As per your update, your gateway IP address should point to the IP address of your router that connects the segments of your network together. It should be on the same network segment as the IP address that the NIC has been issued.
This is set with the Routers option in DHCP (Option 3 I think).