If you need a private snippet, use GitLab snippet instead of GitHub gist because secret gist is not private.
Omg, Really? Yes according to GitHub documentation
"Secret gists aren't private. If you send the URL of a secret gist to a friend, they'll be able to see it. However, if someone you don't know discovers the URL, they'll also be able to see your gist."
I created this secret gist just to show you an example, try to access https://gist.github.com/pvgomes/43e3c575ab401f27937b4426d1e3e815
and this gitlab private snippet https://gitlab.com/snippets/1942790
I really like the way that gist works, but with gitlab we can create a good description of our snippet, this helps also to avoid a lot of comments inside the snippet. It does not mean that gist is bad, it just means that depends of your purpose make sure that you're using a real private snippet.