Gnome has nothing to do with the question.
Kloxo and Webmin might
coexist peacefully, but I doubt it. Most control panels (except for Webmin and Virtualmin) generate their configuration files from templates, and so it's not safe to modify those files outside of the control panel...meaning if you modify something outside of Kloxo in Webmin, Kloxo would probably not respect those changes. Webmin, on the other hand, does respect changes made outside of Webmin; Virtualmin does so, as well, except for a few specific cases.
So, I'd suggest not trying to run both Webmin and Kloxo. And I would strongly suggest not running Gnome at all on a server. X and Gnome serve no purpose on a server, and just waste resources and add further potential vectors for attack (every service you run is another vector for attackers; so don't run anything you don't need, and a server needs X and Gnome like a fish needs a bicycle).
Virtualmin can be found here: http://www.virtualmin.com
and installation instructions here: http://www.virtualmin.com/download
If you did want a web server control panel to run with Webmin, you could run Virtualmin. It is a web server control panel that is built on top of Webmin; so you get both a general purpose systems management UI (Webmin) and a web hosting control panel (Virtualmin) and they work well together because they're developed by the same people. But, if you want Kloxo, you'll pretty much need to just use Kloxo.