Which one? We first had the flurry of third party work (Brook, Lib Sh, etc), then we had AMD "Close to Metal" which was IIRC based on Brook, soon followed with dedicated cards, year later we got CUDA (also derived partially from Brook!) and AMD Stream SDK, later renamed APP SDK. Then we got HIP / HSA stuff which unfortunately has its biggest legacy (outside of availability of HIP as way to target ROCm and CUDA simultaneously) in low level details of how GPU game programming evolved on Xbox360 / PS4 / XBox One / PS5. Somewhere in between AMD seemed to bet on OpenCL, yet today with latest drivers from both AMD and nVidia I get more OpenCL features on nVidia.

And of course there's the part of totally random and inconsistent support outside of the few dedicated cards, which is honestly why CUDA the de facto standard everyone measures against - you could run CUDA applications, if slowly, even on the lowest end nvidia cards, like Quadro NVS series (think lowest end GeForce chip but often paired with more displays and different support that focused on business users that didn't need fast 3D). And you still can, generally, run core CUDA code within last few generations on everything from smallest mobile chip to biggest datacenter behemoth.