eh, Windows Forms is the default GUI library, this means relying on third party libraries if you want to write cross-platform desktop apps.
There isn't really a "default" GUI library at this point. WinForms and WPF are both rather equal in support. (Arguably WPF gets far more updates than WinForms, most months.)
MAUI (in .NET 6+) will be a first party library for some types of cross-platform desktop apps.
The default "GUI" library for .NET Core has always been web.