That was a special promotion with a defined end date. The game is not free. The only legitimate way to obtain it currently is to pay for it. Together with the false claim about HL1 being open source, you're really adding a lot of misinformation to this thread.
It was available for free as part of its 20th anniversary update: https://overclock3d.net/news/software/half-life-2-is-availab...
That was a special promotion with a defined end date. The game is not free. The only legitimate way to obtain it currently is to pay for it. Together with the false claim about HL1 being open source, you're really adding a lot of misinformation to this thread.