Router Port Forwarding – How It Works Without Setting Gateway

There is a device behind the router, say 192.168.0.2, and ssh enabled on port 22. Gw address specified as 192.168.0.1 in the route table.

And there is a router, 192.168.0.1

I'm creating a port forward rule to access the device from wan.

source - port - destination - port
0      - 22   - 192.168.0.2 - 22

So, anyone coming from wan on port 22, can access the device.

However, when I remove the gw address (192.168.0.1) from the route table of the device, then port forward is not working anymore.

What I don't understand is here;

• In local network router and the device can reach each other. If there is an ssh client app in the router, I can connect to the device whether there is a gateway is set in the device or not. So, local network is working on ETH0 interface of the router.
• Port forward should route all my packages coming from WAN interface on specific port to the specific port of specific IP which resides behind the LAN interface.
• Request is coming from the WAN, which means communication is started by a remote device. And in this case, local device should has nothing to do with the gateway address, because it just responding a request which is coming from the LAN interface of the gw.
• When the device sends the response, the router should send it to the remote client.

In this case, I don't see the necessity of entering the gw address to the device.

At which point am I mistaken?