IN DCS GPU usage varies with demand. At high altitudes where there is less dense scenery the usage might drop. When u decent, the gpu usage will increase. I have experienced DCS to be quite complicated when it comes to GPU usage. For example, at an Airfield the fps will drop to some extent and so will gpu usage. Whereas the usage should increase with maintained fps.
I guess its complicated. For example, In the picture u can see both my GPUs at around 50%. When I am at low altitudes at denser scenery, the GPUs go to 100%