Let's see if I understood correctly. You "gateway" is the proxy server, the rest of the network has ip addresses in the 192.168.0.x range, and 192.168.0.100 as their router, right?
The server should route (NAT) the requests to the internet through the ISP router (192.168.1.2). You may only need to route web browsing through the gateway.
In any case, you need to enable ip_forwarding in the gateway, and add a few rules to the firewall configuration.
Please clarify... do you want a transparent proxy?
Pedro.