Any simple examples of using Graph VIew?

To confirm, GraphView is an internal API and not officially supported. It may be removed or changed in the future.

With that said, here's a zip of an old GraphView example project. It's very likely not going to compile in a modern version of Unity but looking at the code may help you figure out how it works. The API itself didn't change all that much since this project was made.

Otherwise, looking at the code for ShaderGraph or Visual Effects Graph would also help if you want to see the API used in a more recent Unity version.