I had recently found this issue wherein the gist was not loading after putting the embed url eg
<script src="https://gist.github.com/dishabhatt123/43e98122304000687293b4c3ef3dc474.js"></script>
directly into the HTML view of my blog. Here is what has finally helped me after grilling down.
Basically browsers does not support random included scripts hence using rawgist. Place the below script on the top of bottom of the page.
<script src="https://rawgit.com/moski/gist-Blogger/master/public/gistLoader.js" type="text/javascript"></script>
Then, add the div as under:
<div class="gistLoad" data-id="43e98122304000687293b4c3ef3dc474" id="gist-43e98122304000687293b4c3ef3dc474">Loading or something, this is just text to display while the browser pulls the gist....</div>
Here, in the above div, data-id ="whereveryourgistidis" and id="gist - whereveryourgistidis"
In the above example of embed url, my gist Id is '43e98122304000687293b4c3ef3dc474'. You can replace it with your gist id from the embed url.
Please Note: you will not see the gist in 'Compose view'. But when you preview your blog, you can see it!!