Old-ish Source has a pretty generous license, no? https://github.com/ValveSoftware/source-sdk-2013/blob/master... GoldSrc is there somewhere as well. I’m definitely grateful for it. I’ve read some of it myself and watched a bunch of Youtube about it.

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.