src/gtk-widgets/grid/ui/MainWindow.blp

using Gtk 4.0;

template $ExampleWindow: Gtk.ApplicationWindow {
  title: 'Python - PyGObject - GTK';
  default-width: 683;
  default-height: 384;

  [titlebar]
  Gtk.HeaderBar header_bar {
    [end]
    Gtk.MenuButton {
      icon-name: 'open-menu-symbolic';
      menu-model: primary_menu;
    }
  }

  Gtk.Grid {
    margin-top: 12;
    margin-end: 12;
    margin-bottom: 12;
    margin-start: 12;
    row-spacing: 12;
    column-spacing: 12;

    Gtk.Button {
      label: 'Button 01';

      layout {
        column: '0';
        row: '0';
      }
    }

    Gtk.Button {
      label: 'Button 02';

      layout {
        column: '1';
        row: '0';
        column-span: '2';
      }
    }

    Gtk.Button {
      label: 'Button 03';

      layout {
        column: '0';
        row: '1';
        row-span: '2';
      }
    }

    Gtk.Button {
      label: 'Button 04';

      layout {
        column: '1';
        row: '1';
        column-span: '2';
      }
    }

    Gtk.Button {
      label: 'Button 05';

      layout {
        column: '1';
        row: '2';
      }
    }

    Gtk.Button {
      label: 'Button 06';

      layout {
        column: '2';
        row: '2';
      }
    }
  }
}

menu primary_menu {
  section {
    item {
      label: _('Preferences');
      action: 'app.preferences';
    }
  }
}