The minimum reputation is 1. You just need to have an account and be logged in for the redirect not to be applied; it only applies to anonymous (logged-out) users:
Starting with the next build, anonymous users will be redirected when they visit duplicate questions with no answers.
As with migration and merge redirects you can disable with ?noredirect=...
Lots of people have Facebook accounts. Most of them are not programmers. The same applies to Google.
GitHub on the other hand is mainly for programmers, and you put code there.
If we put log in with GitHub on Vegetarianism SE or Music: Practise & Theory etc., we'd
Have a feature that would only be used by a small subset of programmers who are also ...
GitHub homepage says clearly:
GitHub is home to the world’s largest community of developers and their projects...
It's for developers, so people who are interested in cooking, or movies, etc... won't really have any interest in it.
Adding the option for GitHub login for non-SO sites will be just noise.