[mepian]

That repository is not the engine, it's just the code for the client.dll/server.dll binaries that contain game-specific logic and some other tools.

[TazeTSchnitzel]

https://github.com/ValveSoftware/halflife on the other hand is the full Half-Life (GoldSrc) engine, but it's not quite an open license.

[jwitthuhn]

That is just the SDK, but there is an unofficial open source implementation of goldsrc.

https://github.com/fwgs/xash3d-fwgs

[manytimesaway]

Also based on leaks/reverse-engineering.

[mepian]

I'm pretty sure this is just the mod SDK as well.