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.
HL2 had both OpenGL and DirectX on paper, but the DirectX implementation was notoriously poor such that everyone used and recommended the OpenGL renderer.
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.
