Is there anyway to slow down, stop, or render DDos and DoS attacks harmless on my servers.
There’s no specific way to. You have to tailor each response to each specific attack. Seen as most GMod servers get hit by devnull attacks, I’ll explain a little. To a generic devnull attack; pure bandwidth is what you need. Block all unused ports and simply eat the attack.
However, if it’s targetted at your gameserver IP, you need to use deep packet inspection to drop all malicious packets. This can be extremely CPU intensive and might do more damage than the attack would itself.
Hardware firewalls are generally not useful; unless you’re willing to pay through the roof for them. Software firewalls are a much better option. On linux, CSF firewall is supposed to be quite good. Snort is another good one. I’m thinking of trialing “InJoy” for Windows; I’ll see how that turns out.