Moinsen Sonic,
the Indigo and other Render Engines don't utilize all hardware on a GPU, there are many parts on such a GPU that are used by video or gaming. The Renderer uses the Compute capabilities only. Change on of the 4 monitors on the top left to CUDA (or one of the Compute Monitors for a AMD Card), and you'll see. Other GPU monitoring tools with show you a 98-100% usage
This is also the reason why a GPU doesn't get that hot/loud while rendering, compared to gaming... in theory you can manually overclock your GPU further for Rendering then for Gaming
