The explanation is a bit technical, but DXVK essentially has a thread that mostly just waits for the GPU to finish executing rendering commands, and measures the time it spends waiting for the GPU, as well as the time spent on doing nothing at all. 100% GPU load means that it always waits for the GPU.
This is not perfect, but quite accurate in practice and works on all systems.
The Mango HUD does not have access to this information and thus cannot trivially implement such a feature. Instead it relies on the GPU load to be reported by your graphics driver, which may or may not work on very old GPUs.
Ok, thanks for your simple explanation.
DXVK_HUDx1 2020-02
There is a new HUD called MangoHud that looks like MSI Afterburner, and one of the things that it can provide is GPU_load, and that doesn't work with old GPUs like mine, but DXVK_HUD shows GPU_load for my GPU, so could you explain what is the method DXVK_HUD use to get GPU_load readings, and if it can be used with other HUDs like MangoHud?