To use the Disqus plugin in your Gitbook project, add the disqus plugin to the book.json file, along with your shortname (you create a shortname for disqus by creating a new website on the disqus.com website)

{
    "plugins": ["disqus"],
    "pluginsConfig": {
        "disqus": {
            "shortName": "XXXXXXX"
        }
    }
}

Then run gitbook install to download and install the plugin.