The documentation on GDK and XSAPI is very sparse on mentions of C#. I can see in NuGet packages from 2017-18 that some of these Xbox Live APIs were available natively, but they seem to have moved away from that way of providing the APIs. Are there more modern dlls for C# included as part of the installation process for GDK or do you have to link against the native C dlls?
I should have found this sooner, but I was thrown off by the documentation calling out Unreal and Unity so did not look further at this section. Microsoft recommends using the packages created for Unity with some minor modifications and integrating them into your project.