From what I can see, gist.github.com uses javascript to embed gists such as:
<script src="https://gist.github.com/1234.js"> </script>And although Google parses some javascript but I doubt 3rd party widgets would get parsed (see update below)
but this exact issue was discussed by Simon Bingham here. He suggests you use the "SEO Friendly Gists" tool which essentially grabs the embed code but ALSO generates a <noscript> version of the gist (that WILL be indexed by google)
Update:
Google Does some Javascript Parsing, and the only official resource I can find on this came from Best approach to have Google index content primarily generated by Javascript where they offer up:
https://developers.google.com/webmasters/ajax-crawling/
Which explains how to have ajax pages indexed.
I've haven't read anything about 3rd party widgets being parsed (nor have I seen any of them own my own sites get parsed) - I think google is focusing on building it's database not improving how your page renders.
If you find some official literature on this, please comment!