1. create layouts/shortcodes folder if not exists
2. add gist.html and past follow code below

   <script type="text/javascript" src="http://gist.github.com/{{ .Get 0 }}.js"></script>

   view raw gist.html hosted with ❤ by GitHub

then you can use it in your .md file by {{< gist embedhash >}}

Reference

• http://blog.cronally.com/embed-gists-with-hugo/