MacBook Pro (16 Inch, 2019) – GPU Memory Continuously at 90% Utilization

Right click on one of the headers and you should see a menu like this:

Click on "% GPU" if it's not already ticked and then sort according to it. That way you should see the process stressing your GPU.

Check the GPU tab in Activity Monitor for apps using the GPU. You'll likely see only the GPUs. Extend the list using the tiny arrow on the AMD GPU and you should see the application using ~90% of your GPU. If you don't see any app using that much, extend the list for you integrated Intel GPU.