Hi Ralf,
there is the 1.9 version only of the Lightbox Gallery published at http://moodle.org/plugins/pluginversions.php?plugin=mod_lightboxgallery
The policy is that the contrib plugin must be published at moodle.org/plugins before its strings are imported into AMOS (one day, this will be done automatically - all contribs in the Plugins database will have their strings registered in AMOS automatically). So unless the 2.x versions of the Lightbox Gallery are not in Plugins.