Gtk.SizeGroup.prototype.set_mode

function set_mode(mode:Gtk.SizeGroupMode):void {
    // Gjs wrapper for gtk_size_group_set_mode()
}

Sets the Gtk.SizeGroupMode of the size group. The mode of the size group determines whether the widgets in the size group should all have the same horizontal requisition (Gtk.SizeGroupMode.horizontal) all have the same vertical requisition (Gtk.SizeGroupMode.vertical), or should all have the same requisition in both directions (Gtk.SizeGroupMode.both).