Well, this is kind of embarrassing. In October, I implemented the GNU Scientific Library as part of the science project I mentioned in the previous post. Since I was working towards an actual deadline, I guess I was too hurried to thoroughly check what licence the GSL uses. Apparently, I assumed it was licensed under the LGPL, which many GNU libraries tend to be. However, it is actually under the much more restrictive GPL, which forbids me from distributing it combined with any proprietary programme. The code given by the Source SDK for the server DLL (the only thing I actually modified) counts as proprietary, since it is distributed under a licence that forbids selling. Therefore, while experimenting with the two pieces of software together is fine, if I were to distribute my compiled server library, I would be in breach of the GPL.
I apologise for not being able to release a compiled game. In order to do so, I would need to either pick a different game engine or a different mathematical library (or write the code myself...). I may do one of those in the future.