Most normal web traffic occurs over ports 443 and sometimes 80. The exclamation point says, “prioritize all traffic from this device IP to the Gaming priority, unless it’s port 80 or 443.” Port 80 and 443 traffic will be processed by it’s normal mark, unless it’s also matching another rule.
Also, remember that the order of the iptables rules is important. The last rule in the list to match is the one which determines the final mark (and therefore priority). So your most important rules should be near the bottom.
Ah ok, this makes sense. Thanks for the info! Seems to be working well for me so far