Gtk.Window.prototype.set_gravity

function set_gravity(gravity:Gdk.Gravity):void {
    // Gjs wrapper for gtk_window_set_gravity()
}

Window gravity defines the meaning of coordinates passed to Gtk.Window.prototype.move. See Gtk.Window.prototype.move and Gdk.Gravity for more details.

The default window gravity is #GDK_GRAVITY_NORTH_WEST which will typically "do what you mean."