“Gists are actually Git repositories, which means that you can fork or clone any gist, even if you aren't the original author. You can also view a gist's full commit history, including diffs.”
? check out the official github documentation
So the key difference is, that they are single files.
Oh, and: gists can be “secret” (as in: private url) also without being a paying github customer, if I understand correctly...