In HWInfo, I only have 1 sensor that shows anything power related for the GPU, which is GPU Core (NVVDD) Output Power. Is this what shows total GPU power and/or TDP. There's no other option for me so I don't really know. I'm on a laptop if that helps.
You could have posted at least a screenshot of the sensors as shown, hard to give advice out of thin air.
I don't think that value reflects the total power drawn by the whole GPU. It most likely covers only the major rails in a constrained (notebook) design and some others are not monitored.