https://help.github.com/en/articles/about-gists#secret-gists
Secret gists
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Secret gists don't show up in Discover and are not searchable.
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Use them to jot down an idea that came to you in a dream, create a to-do list, or prepare some code or prose that's not ready to be shared with the world.
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After creating a gist, you cannot convert it from public to secret.
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Secret gists aren't private
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If you send the URL of a secret gist to a friend, they'll be able to see it.
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However, if someone you don't know discovers the URL, they'll also be able to see your gist.
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» If you need to keep your code away from prying eyes, you may want to create a private repository instead.