Thank you, now it works and I can see the example running.

Terminal command

rdmd -I/usr/include/d/gtkd-3/ -L-lgtkd-3 -L-ldl hello.d 

hello.d

import gtk.MainWindow;
import gtk.Label;
import gtk.Main;

void main(string[] args)
{
    Main.init(args);
    MainWindow win = new MainWindow("Hello World");
    win.setDefaultSize(200, 100);
    win.add(new Label("Hello World"));
    win.showAll();
    Main.run();
}

The result:

For those that do not have D lang or/and gtkD installed:
I tested on another Ubuntu 19.04 system, and it seems that it was pretty straight forward to setup D lang and gtkD for the first time:

sudo snap install --classic dmd
sudo apt-get install libgtkd-3-dev