These 2 simple iptables rules will protect your server from most of the single IP DoS attacks. If your server port is not 27015, then change the --dport parameter in below commands.
Note: Do not ask how to protect server against DDoS attacks. Only your VPS Host Provider can protect your server from DDoS attacks. So contact them or purchase any third party DDoS protection for game servers..
Bash:
iptables -A INPUT -p udp -m udp --dport 27015 -m state --state NEW -m recent --set --name DEFAULT --mask 255.255.255.255 --rsource
iptables -A INPUT -p udp -m udp --dport 27015 -m state --state NEW -m recent --update --seconds 20 --hitcount 20 --name DEFAULT --mask 255.255.255.255 --rsource -j DROP