You can reference Graphics Mill in .NET projects (formerly named .NET Core) due to the .NET Framework compatibility mode introduced in .NET Standard 2.0. You can find more details in the Overview of porting from .NET Framework to .NET.
Graphics Mill has references to System.Drawing and System.Configuration.ConfigurationManager namespaces. Microsoft provides their implementation for the .NET platform. When you install Graphics Mill with a NuGet package, the corresponding packages are added automatically.
Since almost all the functionality is implemented in native code, the working of Graphics Mill in .NET only differs in the following minor aspects: you can't use Desktop Application licensing mechanism and can't serialize objects, because binary formatters became deprecated in recent versions of .NET. Everything else works as expected.
Graphics Mill is still a Windows-only library, and you can't use it on other platforms.