If you want to use C#, the easiest way is probably using GtkSharp. I don't know if there's a way to use the VS GUI designer, but MonoDevelop has a built in GUI designer that works with GtkSharp -- and MonoDevelop runs on Windows too.
Might be worth trying out. You can still run your application on Microsoft's .NET stack on Windows, GtkSharp supports it just fine, and because it uses GTK it will look native on all of the supported platforms.
Disclaimer: I haven't actually tried any of the above.
Might be worth trying out. You can still run your application on Microsoft's .NET stack on Windows, GtkSharp supports it just fine, and because it uses GTK it will look native on all of the supported platforms.
Disclaimer: I haven't actually tried any of the above.