The most interesting thing about this link, for me, is seeing that there's an open-source recreation of GoldSrc [0]? That's apparently been around since 2011 [1]? I'm stunned I haven't heard about this before. Very cool.

[0] https://github.com/fwgs/xash3d-fwgs

[1] https://www.moddb.com/engines/xash3d-engine

Xash3D is not really open-source. The original codebase comes from leaked/decompiled code, and nowadays still requires the proprietary HLSDK to build.