I can see how to view a specific revision and all revision diffs for a GitHub Gist, but do not see how to pick two revisions for comparison, like Wikipedia does. How can this be viewed?
There is currently no way via the GitHub web interface to see the diff between any two points of your gist revisions.
The alternative is to download the Gist and then reupload it to your main GitHub account as a full repository and then modify the URL as needed, like the following:
You can also use the Diff for gist.github Chrome extension.