Github announced that in 30 days they will no longer allow anonymous Gist creation any longer. The reason they stated they will be deprecating this is that they receive a huge amount of spam.
In 30 days, we’ll be deprecating anonymous gist creation—a decision we made after a lot of deliberation. Anonymous gists are a handy tool for quickly putting a code snippet online, but as the only way to create anonymous content on GitHub, they also see a large volume of spam. In addition, many people already have a combination of tools authenticated with GitHub that allow them to create gists they own.
Most developers will have a Github account already and you’ll still be able to use Gists as long as you are logged in. This should have little effect on most people unless you are trying to upload something and remain totally anonymous. Of course, other services exist for that and would be better suited.