Could be even more basic, but this showcases some of the features the Vala language.

The code

using Gtk;

int main (string[] args) {
    Gtk.init (ref args);

    var window = new Window ();
    window.title = "First GTK+ Program";
    window.border_width = 10;
    window.window_position = WindowPosition.CENTER;
    window.set_default_size (350, 70);
    window.destroy.connect (Gtk.main_quit);

    var button = new Button.with_label ("Click me!");
    button.clicked.connect (() => {
        button.label = "Thank you";
    });

    window.add (button);
    window.show_all ();

    Gtk.main ();
    return 0;
}

All GTK+ classes are inside the Gtk namespace. You must initialize every GTK+ program with Gtk.init ().

Compilation and Running on Linux

$ valac --pkg gtk+-3.0 gtk-hello.vala
$ ./gtk-hello

This needs the valac compiler, gcc, the glib and gtk3 development packages installed on your system.

Taken from the GNOME Wiki.