It does not work that way. A NAT forwards to a target IP. However in most situations, if the game is talking to a server somewhere else, the client initiates the connection and the router will forward responses to the IP that originated the request. No special setup is normally required. If you are setting up a game server that other people will connect to, you need to set up port forwarding for the specific ports or port ranges to that server. These are in general terms since you still are being very vague. However don't bother telling me what game it is as it would be meaningless to me.
Wrong. Author of the topic wanted to open ports to his client side, in order to game servers from outside could make a connections to him in order to provide connectivity to other users.
To author - you need to get ports of games you're gonna use and forward these to your devices.
In order to pass though the NATed traffic from your first router, you don't need to build second nat rules on your second router, you just need to setup simple routing between them: after dstnat work, fist router just have to know a route to a host you want provide traffic to. After routing decision, packet with source IP of server and destination of needed 192.168.*.* host, will flow to it. If you have more questions, just ask here.