Generally we give content to github API which in response gives user a gist with a unique ID. Is it possible to provide gist ID by user and then create a gist?
From the GitHub API on Gist, the gist ID seems always generated by GitHub in response of gist creation.
As a result, it doesn't seem to be customizable.
It might have been using a short SHA1 in 2014, but quickly used a longer SHA1 since then.