AFAIK .NET Core offers nothing of any value for cross-platform GUI though. There is Xamarin.Forms but last I looked at things it was pretty shit compared to Java's offerings.
Xamarin.Forms isn't really equivalent to any (commonly used) Java options. Something like GTK# or WinForms+Monk would. There's also Eto and Avalon "in the pipeline" so to speak.
To me neither Java nor C# does well at WORA UIs but they are at parity imo. JavaFx isn't bad, but it doesn't do anything the C# offerings can't do fairly easily.
I'm actually a fan of Qt for true cross-platform apps these days myself