GPU encoders basically all run at the equivalent of “fast” or “veryfast” CPU encoder settings.
Most high quality, low size encodes are run at “slow” or “veryslow” or “placebo” CPU encoder settings, with a lot of the parameters that aren’t tunable on GPU encoders set to specific tunings depending on the content type.
It’s used for the majority of HDR streams and all Dolby Vision streams. h265 is the only codec that supports DV