The main game page doesn't list mac as a compatible operating system, just windows. I just realized it was available for mac by accident while looking through my library. How long has this been the case?

The original strike suit zero was ported to Mac and Linux (Unfortunately for linux users, GOG never received linux build). The Directors cut was always Windows (7 SP1 64-bit and up, original worked with 32-bit Vista and up) only (the DC version iirc is actually port from console version). However GOG version of DC has original version along with it's DLC as free extra both for windows and Mac. This is why GOG can't advertise DC as being Mac compatible.

Oh, okay. That makes sense.