動画デコードには勿論GPUが使われている。たとえばこれは記事の一部だけど…
1080p H.265の動画再生が出来るのもMali T760のGPUのおかげだよ。

The RK3288 is based on the ARM Cortex A17 architecture and it’s a quad core setup with speeds up to 1.8 GHz.
As a GPU we have the Mali T760 graphics chip which is scalable up to 16 cores,
with H.265 hardware decoding and also support for 4K x 2K resolution.