If you enter an URL that is not under github.com domain, git.io will warn you that URL must be from github.com.
One more thing to note here, besides changing
.com is that if you create a short URL for a GitHub URL without providing a custom code argument, you won't be able to use Git.io to shorten the same GitHub URL again with your custom code.
However, I used curl to shorten the same URL again (this time without the "S" in "HTTPS" so it's a different URL) and provided a "code" argument:
curl http://git.io/ -i -F "url=http://1oh1.github.com" -F "code=1oh1"
That got me what I wanted:
HTTP/1.1 100 Continue HTTP/1.1 201 Created Server: GitHub.com Date: Wed, 17 Sep 2014 08:30:39 GMT Content-Type: text/html;charset=utf-8 Connection: keep-alive Status: 201 Created Location: http://git.io/1oh1 Content-Length: 23 X-Runtime: 0.008799 X-Node: gitio2
Use your normal github.io address but change the ".io" to ".com" (e.g. gabrielecirulli.github.io/2048/ would be entered as gabrielecirulli.github.com/2048/).
Here's an article about the githup pages that explain the github.io vs githum.com relation as well: https://github.com/blog/1452-new-github-pages-domain-github-io