I could be wrong (I never really got into gaming as GMA950+Linux was an exceptionally bad combo for it) but I thought source was exclusively DirectX back then.
https://arstechnica.com/civis/viewtopic.php?f=22&t=1007962
https://arstechnica.com/civis/viewtopic.php?f=6&t=931690
https://arstechnica.com/civis/viewtopic.php?f=22&t=987307
Ironically now with Valve's push for Linux gaming, I'd expect that Linux is probably a great platform for HL2, probably even better than Windows. Intel GMA drivers might still be bad though, not sure if that is too far back for the open-source driver that showed up in the Core era.
Edit: I'm thinking of the GoldSrc engine used in Half-Life