I have got to the bottom of this problem and I think it is probably purely the Debian packaging that is faulty.
I'll document it on this mailing list in case anyone else finds it useful, but I should probably let the Debian package maintainers know. It is confusing for a new user who is following the documentation on GNS on the gnunet web site.
The plugin objects required to use the GTK GNS manipulation screen (e.g. libgnunet_plugin_gtk_namestore_a.so etc) are only supplied in the Debian gnunet-gtk-dev package, which isn't installed by default with gnunet-gtk. What is more even if you do install the dev package you must then manually create symbolic links in /usr/lib to point to these objects as they are not picked up, because they reside in /usr/lib/gnunet.
So if you install the dev package and add the sym-links, the GUI GNS screen allows you to add new GNS records: It then works correctly.
Regards
Geeb