The classic game from Valve is free to download and play on macOS with a caveat.
Half-Life turned 25 in November, and to commemorate the occasion Valve is giving away the game for free for a limited time. Steam users will be able to add to their accounts and download to play until November 20. Users will be able to keep the game as long as they claim it before the deadline.
Mac players may need to complete a few steps before they can start playing Half-Life 2. Valve states that the game is not compatible with macOS Catalina 10.15 or above due to ending 32-bit support with newer operating systems. Several workarounds include using Boot Camp, using an APFS volume, or a Mac running the older operating system. Minimum system requirements are quite easy to meet, as the game is old and can run on 1GB memory and at least an ATI X1600 or Nvidia GeForce 8.