The beta version of S&Box (Garry's Mod 2 / GMod 2) is not available for download. Rumors speak of a release date in 2019.
The studio has not yet communicated on supported platforms, either for the client version or the server version.
We will keep you informed as soon as the studio will communicate on the release of the beta version and the different ways to download the game.
To know more, please visit the official website!