I have been developing Windows Forms programs for few years. I am now looking into .NET Core (including ASP.NET Core MVC). I am searching for the new GUI desktop technology.
Windows Forms (and its visual designer) have been available for .NET Core (as a preview) since Visual Studio 2019 16.6. It's quite good, although sometimes I need to open Visual Studio 2019 16.7 Preview to get around annoying bugs.
See this blog post: Windows Forms Designer for .NET Core Released
Also, Windows Forms is now open source: https://github.com/dotnet/winforms