This plugins requires gitbook >=2.0.0.
Add this to your book.json, then run gitbook install:
{
"plugins": ["url-embed"]
}
The plugin allows you to embed dynamic content in your Gitbook as an iframe, and if you generate a PDF or ePub version of the book, it will instead insert an image of the corresponding content.
The plugin expects that you will have a .png image in your /assets/ folder with a name that matches the path of the url. For instance:
{% urlembed %}
https://website.org/stuff/this-is-the-path-name
{% endurlembed %}
For the example above, you should include a file in /assets/ called 'this-is-the-path-name.png'