OK, a google search gave me the answer:
http://www.webupd8.org/2010/04/gthumb-211x-removed-from-ubuntu-1004.html
I quote:
"An update in Ubuntu 10.04 Lucid Lynx yesterday reverted gThumb from 2.11.x
to 2.10.11:
Revert to 2.10.11. It appears that 2.11.x series is not stable enough for an
LTS with too many big changes. The new workflow isn't documented enough as
well. (LP: #545871)
The package in Lucid is now called "gthumb_2.11.2.1.is.2.10.11-0ubuntu1".
This is a big hit to gThumb which adds a lot of amazing features in the
latest 2.11.x series, such as Flickr and Picasaweb export support. Even more,
the latest GIT build also includes a new extension to export photos to
Facebook."
