S&box is not going to be Garry's Mod 2. It will eclipse what is possible in Garry's Mod rather than simply be a modern version of it.

We are taking Source 2 and using our knowledge of Source 1, Garry's Mod, Unity and Unreal Engine we're trying to create an intuitive, modern feeling game development environment where everything is moddable.

Like Rust and Garry's Mod - we see this as a long term thing. We intend to eventually make our own games using it. It is something for us to work on and improve every day for the next couple of decades.