Hello FredR,
Currently the following stages support GPU-based computations:
- image matching (includes feature points detection, preselection and matching itself),
- depth maps calculation (first phase of build dense cloud and build mesh in experimental mode),
- refine mesh operation.
All other stages currently involve CPU-processing only.
So you should check GPU activity only during these stages. To do that I could suggest to use dedicated utilities, like GPU-Z for example, also you can access the current GPU load via NVIDIA SMI utility for your card.
As for Windows 10 Task Manager, I think that the may graph is showing some average percentage across different possible activities, most of which are not utilized by PhotoScan at all, so you may need to open Compute_0 or Compute_1 graph in Task Manager to see the actual activity.