How can I read Nvidia GPU usage in C#?

It's a little difficult with c# alone, and you have to use c/c++.(Of course, if you need to compile the source, and if you have nvapi.dll, you don't need it. It's probably in the directory of the nvidia program. I'm not currently in Windows, so it's hard to check.)

First of all, it would be most accurate to use NVAPI, the NVIDIA's core software development kit.

https://developer.nvidia.com/gameworksdownload#?search=nvapi

I found out that there is already a project to wrap nvapi.dll with c#.

https://github.com/falahati/NvAPIWrapper

You can add the assembly with nuget and look at the sample.