I won’t be much help for your PC gaming. My advice is to identify the traffic and then build outbound traffic policies that make sense.
You can identify the traffic by using the search function (Status -> Active Sessions -> Search tab) or packet capture feature (on support.cgi).
Once you identify the traffic, we can help you build outbound policies. I would imagine that the game downloads are still over port 80, and the games themselves are UDP – but, you will have to figure that portion out.
Here is what I would do… Go to Status -> Active Sessions->Search. Enter the PC that is doing the gaming in the source IP, then change the subnet mask to 255.255.255.255 (/32). Change the protocol to ALL. That will restrict the search results to ONLY his PC. Then load the game up and see what new sessions show up.