It appears that the game engine you are using (you didn't specify) may be using UDP and not TCP so the usual Install Fail2Ban and mod_security (listening on port 27115) then write a mod_security filter to trigger on the problematic "TSource Engine Query" requests if you can.... probably wouldn't work here.
That said you could easily do almost the same thing using Snort and Fail2Ban
There are other things you can do as well:
See if the IP's are associated with a particular BGP AS number, country, ISP, or possibly with Tor exit nodes (these can be blocked).
If the system(s) processing these specific requests can be isolated to a separate set of computers, such that other services won't be affected when this happens again, do so.
Do preventative performance tuning on your systems so they handle well under load. Identify key bottlenecks and push as much as possible into RAM including your database.
Look closer at the packets and see if there are any unique characteristics about the clients making these requests that might allow for easier and safer blocking.
Some anti-DDoS tools only work for specific protocols (TCP and not UDP) and/or only for certain ports (80/443), make sure you understand how these work and don't work. It may not be your ISP's fault but simply a case that the tool they have only helps if it's a website or a service running on TCP.
Do normal clients connect with a specific request before making additional requests? If so you may be able to use a server-side cookie (or at least leverage that equivalent action) to authenticate the type of user before allowing certain types of queries to be performed and block all others (Think: is this a "Human or valid client" cookie).
If there is any initial web component and it's an option, use a tool like reCaptcha when users initially arrive at the site to verify they aren't part of the attacking bots.
The best solutions will require more security architecture but these are good places to start. Snort combined with Fail2Ban and IPTables may be your best option for a quick solution for now.