Is [github.com/Microsoft/GSL](https://github.com/Microsoft/GSL) the GSL?

No. That is just a first implementation contributed by Microsoft. Other implementations by other vendors are encouraged, as are forks of and contributions to that implementation. As of this writing one week into the public project, at least one GPLv3 open-source implementation already exists. We plan to produce a WG21-style interface specification to ensure that different implementations agree.