Note: This tool, despite its name, is not an official Greasemonkey API.
The purpose of GM_config is to create a lightweight graphical settings dialog that can be easily used in user scripts, through @require.
To use this tool in your script, simply include in the Metadata Block, for example:
// @require https://openuserjs.org/src/libs/sizzle/GM_config.js // @grant GM_getValue // @grant GM_setValue
For detailed usage and reference, please see the "wiki" linked below.