This simple WordPress plugin will allow you to paste Github Gist urls directly into your WordPress post editor and they will display in the Gist codebox automatically for you in your site's content.
Paste a gist link into a WordPress post or page and it will be automatically embeded for you.
https://gist.github.com/stuartduff/7128278
If your gist has multiple files you can choose only a single file by using the following URL format in your WordPress post or page.
https://gist.github.com/stuartduff/7128278?file=embed-gist.php