I can access that Gist without being logged in to a GitHub account.
When I was at our local hackerspace, it was demanding that I log into GitHub. When I tried other gists, it did the same.
Now, at home, I'm seeing the content. I can only chalk this up to a one-off weirdness that I just happened to run across.
Someone sent me a GitHub gist in Discord, and it wouldn't let me see it until I logged in. I clicked the link again, and it let me through just fine. A/B testing maybe?