Welcome!
There are already several tutorials on how to GUI apps in system containers. I would suggest to consult them. Doing this through VNC for games may be painful. But if you insist, then you would just need X11 forwarding when you SSH to the system container. It’s a parameter in SSH to forward for you the Unix socket for the X server. Then, you run an X11 app in the container and you get the output on your desktop.
An improvement is to setup X2Go. With this you you will get a whole desktop session in the container, and on your desktop you would get a window of that desktop session. Really cool, download:start [X2Go - everywhere@home]
If you want to do this with GPU acceleration and run games decently, see this guide, Incus / LXD profile for GUI apps: Wayland, X11 and Pulseaudio