For that, u se Mercury Playback Engine Software Only mode.If you don't have a supported GPU, you can still use Premiere Pro you just won't get the advantages of processing with CUDA, OpenCL, or Metal.In some cases, it can actually mean that results are better, as with scaling. Processing with CUDA, OpenCL, or Metal doesn't just mean that things are faster.Error Compiling Movie errors are often at the root this issue. Note that whether a frame can be processed by CUDA, OpenCL, or Metal depends on the size of the frame and the amount of VRAM on the graphics card.The three technologies do work in concert, with default presets for hardware acceleration set accordingly.GPU acceleration is also available for encoding of H.264 and HEVC formats regardless of CPU type.GPU acceleration is available for decoding and encoding of H.264 and HEVC formats with certain Intel GPUs using Intel Quick Sync.That said, two new options for GPU accelerated encoding have been added recently, both of which use different tech than the Mercury Playback Engine: One set of things that Premiere Pro's Mercury Playback Engine doesn't process: encoding and decoding.Here's a list of things that Premiere Pro can process with CUDA, OpenCL, and Metal: The official and up-to-date list of the cards that provide CUDA, OpenCL, and Metal processing features is here. 4GB for 4K for the Mercury Playback Engine to function as specified. For current versions of Premiere Pro, you need at least 2 GB VRAM for HD.Then, OpenCL and Metal processing for the Mercury Playback Engine came along. CUDA support on Nvidia GPUs came first.Since then, GPU Acceleration was added, enhancing Mercury Playback Engine performance.All versions of Premiere Pro (since CS5) have the first two of these: 64 bit and multi-threaded applicaton.

See Adobe documentation on the topic here: Mercury Playback Engine (GPU Accelerated) renderer. Mercury Playback Engine ExplainedMercury Playback Engine is the given name for a group of improvments that is been built into Premiere Pro.

Metal supported gpus