Why is the GPU utilization low?

To add to David's response, you'll see the GPU kick in if you're using Adobe Media Encoder to render Premiere Pro Sequences or After Effects Compositions that use Effects that are GPU accelerated.  

For more, check out the link that he posted to GPU Accelerated Rendering & Hardware Encoding and click through the Q&A under Mercury Playback Engine (GPU Accelerated) renderer.

Video encoding doesn't use the GPU a lot, however, earlier this year Premiere and Media Encoder added hardware-accelerated H.264 encoding via Intel Quick Sync: https://helpx.adobe.com/x-productkb/multi/gpu-acceleration-and-hardware-encoding.html

Your Core i9 does support Quick Sync, so check out the support article and make sure in version 14.7 you have Hardware Acceleration enabled. This is a dedicated piece of hardware on your CPU, so your GPU utilization won't be affected here. Just because you have a good GPU doesn't mean that it can be used, or should be used, in every circumstance. It also doesn't mean that software can't be written or updated to take more significant advantage of your hardware, but it's not nearly as simple as saying "throw it on the GPU." Once hardware accelerated H.264 encoding was introduced, my encoding time sped up on average 3-4 times.