CUDA, OpenCL, Metal, or OpenGL?

The GPU architecture of Premiere Pro is entirely CUDA/OpenCL/Metal, and this is what is exposed through the GPU extensions to the effect/transition APIs. A GPU-accelerated effect may use any combination of CUDA, OpenCL, and Metal.

If you want, you have the ability to transfer frames from CUDA/OpenCL to OpenGL (though not always efficiently). Read more about that here.