Messed up that first one
body.html
— SUCCESSNote: adding the image as image.gif truncated the files after it (index.html and readme.md) on the gist page, so I renamed it to z.gif which resolved the issue
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<html>
<head>
<meta charset="utf-8">
<title>Relative gist files</title>
<script src="//cdnjs.cloudflare.com/ajax/libs/jquery/2.1.1/jquery.js"></script>
<script>
$(function () {
$('body').load('body.html');
});
</script>
</head>
<body>
</body>
</html>
https://cdnjs.cloudflare.com/ajax/libs/jquery/2.1.1/jquery.js