It was not exactly obvious. Here's how to revert a Gist commit!
Checkout the gist like a normal git repo:
# replace the Gist ID with your own
git clone git@github.com:cc13e0fcf2c348cc126f918e4a3917eb.git
Treat it like a normal repo. Edit, force push, etc.
When you are ready, you will need to force push as the remote has newer commits:
git push -f origin master
Want to revert the last change?
git reset --hard HEAD~1 && git push -f origin master
Yay! You reverted!