That doesn't use the GPU cores. They just happen to package dedicated video hardware alongside it, but it's more similar to a CPU with a specialized DSP attached.

Any kind of decompression isn't fully parallelizable. If you've found any opportunities, that means the compression wasn't as efficient as it theoretically could be. Most codecs are merciful and eg restart the entropy coder across frames, which is why the multithreaded decoding in ffmpeg is able to work.

(But it comes with a lossless video codec called ffv1 that doesn't allow this.)