Note: This tool, despite its name, is not an official Greasemonkey API.
The purpose of GM_config is to create a very lightweight graphical settings dialog that can be easily used in user scripts through @require. A scriptwrighter using GM_config will pass an JSON object that contains the information about the values to be stored. 
To use this tool in your script, simply include in the Metdata block:
// @require http://gmconfig.googlecode.com/svn/trunk/gm_config.js
For detailed usage and reference, please see the "usage guide" linked below.