GTK+ windows by default are decorated by the window manager. You can request the titlebar of an application be replaced in GTK+ 3.10 or higher by calling
gtk_window_set_titlebar(window, header);
where header
is a GtkHeaderBar
widget and window
is the window you wish to have client side decorations. The official GNOME documentation has more information on this subject. As far as I gather the window manager is under no obligation to respect this request and can continue decorating it in the server.