When you use _Render you’ll be using the old-style renderer, which is not using the GPU.
You’ll need to use the Raytraced viewport mode to utilize your GPU. To get render results from the Raytraced viewport you’d use the _ViewCaptureToClipboard and _ViewCaptureToFile commands. If you are working with more raytrace-intensive scenes (size-wise, or complexity otherwise), you might want to consider using _TestPackageManager and install the rhino-render-next package, which gives you _Render capability with the GPU using the Rhino Render Next renderer (which is Raytraced, but then for _Render).