A secret gist should be used for anything that you don't want to appear publicly on GitHub. Secret gists can be used when you don't want private keys to be accessible to the public, or for and private code in general.
A simple example of JSON code that would be better fit for a secret gist:
{
"id": AKIAIOSFODNN7EXAMPLE,
"secret": wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY
}