First of I would like to thanks disqus.com for providing an awesome service to all bloggers and website makers. Adding comments to your google site is so easy just follow the steps mentioned below.

1)     Login to disqus.com

2)     Select your plan as per your requirement (Paid/Unpaid basic options are available)

3)     Add your website name

4)     Select Platform (For google sites use Universal code

5)     Get your code from disqus as below and do necessary changes

<div id="disqus_thread"></div>

<script>

/**

    *  RECOMMENDED CONFIGURATION VARIABLES: EDIT AND UNCOMMENT THE SECTION BELOW TO INSERT DYNAMIC VALUES FROM YOUR PLATFORM OR CMS.

    *  LEARN WHY DEFINING THESE VARIABLES IS IMPORTANT: https://disqus.com/admin/universalcode/#configuration-variables */

/*

var disqus_config = function () {

    this.page.url = PAGE_URL;  // Replace PAGE_URL with your page's canonical URL variable

    this.page.identifier = PAGE_IDENTIFIER; // Replace PAGE_IDENTIFIER with your page's unique identifier variable

};

    */

(function() { // DON'T EDIT BELOW THIS LINE

var d = document, s = d.createElement('script');

s.src = 'https://XXXXXXXXXXXXXXXX.disqus.com/embed.js';

    s.setAttribute('data-timestamp', +new Date());

    (d.head || d.body).appendChild(s);

    })();

</script>

<noscript>Please enable JavaScript to view the <a href="https://disqus.com/?ref_noscript">comments powered by Disqus.</a></noscript>

 Edit the RECOMMENDED CONFIGURATION VARIABLES section using your CMS or platform's dynamic values.

You are done. You can moderate the comments from your DISQUS site.

Thank you.