Looking at moski's blog, his description and gist snippets (gistLoader.js and gistBlogger.js), I can suppose that to reach your goal you have to edit that code a little bit.
Currently, when you add
<script src="https://raw.github.com/moski/gist-Blogger/master/public/gistLoader.js" type="text/javascript"></script>
at the bottom of your posts, what this script does is looking for this other code you added into your blog
<div class="gistLoad" data-id="GistID" id="gist-GistID">Loading ....</div>
retrieves the data-id attribute, and injects the required code to load the script with src set to
'https://gist.github.com/' + id + '.js'
Now, if I correctly figured out what the code does, editing the second moski's HTML code in  this way:
<div class="gistLoad" data-id="GistID" data-file="GistFile" id="gist-GistID">Loading ....</div>
and the function in moski's gistBlogger.js in order to retrieve (when defined) the new data-file attribute, you can generate a new src to inject, like that:
'https://gist.github.com/' + id + '.js?file=' + file
It should works.