We've been playing with it for a long time. It started off on the Unreal Engine, but now it's being developed on Source 2.
It's 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 enviroment where everything is moddable.
Like Rust and Garry's Mod - we see this as a long term thing. Something for us to work on and improve every day for the next couple of decades.