I don’t know any details about your workload but note that the CPU will still be used even if the actual kernel execution is done on the GPU. In this case the CPU would still be at least responsible for the kernel launches and usually also for the data loading and processing.
To properly see the GPU utilization in the Windows task manager you would have to select the “compute” tab (in case that’s not already done) as the default view apparently does not show the utilized compute resources (we’ve had similar discussions about it e.g. here).