My company uses GitHub Enterprise. Some people are creating Gists to share simple codes. I searched, and it is not possible to create "organization gists".
What will happen to those gists if the person who created them leaves the company (and is no longer listed/member of the GitHub Enterprise)?
As outlined in the documentation,
If employees leave the company, you can suspend their GitHub Enterprise Server accounts to open up user licenses in your GitHub Enterprise license while preserving the issues, comments, repositories, gists, and other data they created. Suspended users cannot sign in to your instance, nor can they push or pull code.
The key part of the above quote is "preserving the issues, comments, repositories, gists, and other data." Their Gists remain in GitHub Enterprise Server, even though their owner can no longer access them.
While Gists are git repositories, there is no way to transfer ownership of a Gist. Instead, you can fork and clone Gists or use a technique like this to move the Gist to a regular Git repository.