Thanks for posting about this. I currently store my TODO lists in Markdown documents within a Github repository and was disappointed to find out the Task Lists feature wasn't available there, rather only in the body of Github Issues.
It's great to see support now built into Gists. I imagine I'll now move my TODO lists into one or multiple Gists in future.
A point of clarification that wasn't immediately obvious from the linked blog post, but for this feature to work, the file extension for the Gist must be set to .md.