You cannot assign a gateway that is not on the local network, through route add, or any process that i know of.
1) You'll need to add a second NIC, placing this on the 192.168.2.0 network (assuming a /24 subnet) with a default gateway on that NIC to 192.168.2.1 for your internet traffic. Also clearing any default gateway that was setup on the 192.168.200.0 NIC, since all the resources it needs to access are local to it on the 192.168.200.0 network it doesn't need a route to them.
2) If you dont want to add a second nic, you'll need some form of a routing device (router or server) that can route traffic between the 192.168.200.0 network and the 192.168.2.0 network. For those devices on 192.168.200 0 their gateway would be this devices 192.168.200.x ip. this device would then have a default gateway to 192.168.2.1 so all internet traffic gets routed to your external gateway. Note for internet traffic to get back to the 192.168.200.0 netowork your external gateway will need a route pointing traffic destined to 192.168.200.0 to this new gateway devices 192.168.2.x ip address