If you are using a Unifi Security Gateway, you can assign bandwidth control per networks, so you can limit the guest wifi to whatever you want per kb/s.
Although, it would just be easier to block every port except the one you need. I don't remember if you can do that with the Unifi hardware (although I would be surprised if you can't with a USG) but you should be able to do that with any business/enterprise class firewall. THere will be backlash but there's nothing you can do about it, there's no way to figure out all the ports used per game - there's just too many ports and games.
EDIT: And from there it's starting to sound like it's going to turn into another full-time job for you to support it all.