From gdm-screenshot.c:main()

        /* Option parsing */
        ctx = g_option_context_new (_("Take a picture of the screeen"));

Should use N_() here; the set_translation_domain call below takes care of translation.

        g_option_context_set_translation_domain (ctx, GETTEXT_PACKAGE);
        g_option_context_add_main_entries (ctx, options, NULL);
        g_option_context_parse (ctx, &argc, &argv, NULL);

Fails to check for errors from parsing.

        g_option_context_free (ctx);

        gtk_init (&argc, &argv);

Probably, instead of this extra gtk_init call, you should simply add the gtk option group to the option context above with

   g_option_context_add_group (ctx, gtk_get_option_group (TRUE));