I like activity or usage because it doesn't imply the GPU is running at full "load" which kind of means something different IMHO. I say this because different workloads might not utilize every shader or every clock cycle as another would while still using all of the GPU's resources for whatever this is actually measuring (CU usage?) For example, your GPU is going to run a lot hotter running Furmark than it is running Heaven Valley benchmark. While both are fully utilizing the GPU in terms of available compute to to be used, Furmark is using far more resources in terms of shaders on each CU, even if both are fully utilizing all of the CUs in total.
TL;DR: You can utilize all of the CUs without using all of the shaders, so I vote for usage as it *could* still be a bottleneck at 100% and not be fully "loaded".
In reality, this is all just semantics if you think about it but, "Usage" is probably the most accurate given what it's describing.