Based on how you're observing the loading move from 100% CPU ro 100% GPU, I would suggest that it is "working" to some extent.
I don't have any experience with that GPU, but here's few things to keep in mind with this:
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When you use a GPU for video encoding, it's not the case that it's 'accelerating' what you were doing without it. What you're doing is switching from running a software implementation of an HEVC encoder on your CPU to running a hardware implementation of an HEVC encoder on your GPU. Hardware and Software encoders are very different to one another and they won't combine forces; it's one or the other.
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Video encoders have literally hundreds of configuration options. How you configure the encoder will have a massive impact on the encoding time. To get results that I'm happy with for archiving usually means encoding at slower than real-time for me on a 5800X CPU; if you're getting over 100fps on your CPU I would guess that you have it setup on some very fast settings - I wouldn't recommend this for anything other than real-time transcoding. Conversely, it's possible you have slower settings configured for your GPU.
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Video encoding is very difficult to do "well" in hardware. Generally speaking software is better suited to the sort of algorithms that are needed. GPUs can be beneficial in speeding up an encode, but the result won't be as good in terms of quality vs file size - for the same quality a GPU encode will be bigger, or for the same file size a GPU encode will be lower quality.
I guess this is a roundabout way of suggesting that if you're happy with the quality of your 100fps CPU encodes, stick with it!