$conf['allow_html_descriptions'] = true;

You don't already know how to change all parameters in PhpWebGallery.
It's very simple.
You need to create a simple php file in the include directory.
./include/config_local.inc.php

There you can set all $conf which are already set in the config_default.inc.php.
In the config_local you only need modified parameters because the local file override the default one.

To avoid any problem, we recommend to install the last release of LocalFiles Editor

Out of this, 'allow_html_descriptions' is true by default;
and BBCode support is available via another plugin - a bugged plugin for 1.7.1 - correct if you have 1.7.0 - BBCode_Bar

Enjoy!
8-)