I think "Secret Gist" is just a renaming of "Private Gist" which has existed for a while: only people with the url will see it. It's a better and more accurate name, for sure.

I do wish there was a true "private" gist, though, for easily preventing people I don't specify from seeing it. I'd love to be able to use it to trade code examples for work, but there's nothing to prevent someone from accidentally pasting the url into the wrong irc window and showing the world our internals.