This is the standard public/private mechanism for most all pastebin sites. Private gists are private by the virtue of the fact that the URL is more or less unguessable. It's not actually authentication-restricted.
For example, pastie does this. Here's a public paste:
And I can see the public paste made before mine by decrementing the URL:
But I can make a private paste:
http://pastie.org/private/sd1fh4m2zesdn6nzkpcfa
And the URL is a value that's likely some sort of ID + salt digest, and has enough entropy that I couldn't effectively guess any other private URL.
Pastebin sites (Gist included) are specifically for the purpose of sharing code with other people. It's not intended to be used as a private clipboard for you to save snippets for later reference, though that's certainly a valid use of it, as long as you're aware that they aren't actually authentication-restricted.