GitHub's spec here is based on [CommonMark][1], which has been around for a while now, and [was originally authored][2] by a group of representatives from GitHub, Reddit, and Stack Exchange.
The first two. [The GFM spec][1] is literally just CommonMark with a few extra extensions added. They even highlighted the new sections green in the spec to make it clear where the GFM spec differs from CommonMark. Everything else is word-for-word identical.
Some quick reading of the linked article says this spec provides a few optional, superset features on top of CommonMark, that it did not contain (like tables, etc).
[1]: http://commonmark.org/
[2]: https://blog.codinghorror.com/standard-flavored-markdown/