https://github.com/ValveSoftware/halflife on the other hand is the full Half-Life (GoldSrc) engine, but it's not quite an open license.
That is just the SDK, but there is an unofficial open source implementation of goldsrc.
https://github.com/fwgs/xash3d-fwgs
I'm pretty sure this is just the mod SDK as well.
That is just the SDK, but there is an unofficial open source implementation of goldsrc.
https://github.com/fwgs/xash3d-fwgs