How do I implement Google+ comments on a Craft site?
This is a bit tricky, and there may be some tradeoffs that I'm unaware of, but the following code works for me:

&lt;script src="https://apis.google.com/js/plusone.js"&gt;&lt;/script&gt;

&lt;div style="width:100%"&gt;
  &lt;div class="g-comments"
       data-href="{{ entry.url }}"
       data-width="675"
       data-first_party_property="BLOGGER"
       data-view_type="FILTERED_POSTMOD"&gt;
  &lt;/div&gt;
&lt;/div&gt;


Two things to note about the code above. 


You want to pass the url that will be associated with the comments to the data-href attribute. The example above assumes you are on a page where {{ entry.url }} is the value you want.
The line that references BLOGGER is required: data-first_party_property="BLOGGER"


I can't explain why that is, but the odd specificity of it may also be a reason why this isn't an easy thing to figure out how to do, as it may suggest this is an unsupported method of adding Google Comments to your site or that the method to add Google Comments to your site will change at some point.

In my searches however, this was the only method I found to do it at all and it seems to be used by several people and work just fine at the time of this post.