S&box is the upcoming spiritual successor of Garry's Mod.
S&box is a game developed by Garry Newman (Creator of Garry's Mod and Founder of Face Punch Studios) uses Source 2 as it's Engine.
S&box has all (almost) tools that Valve used to make Half-Life:Alyx.
unlike Garry's Mod if you wanted to make a Add-on(s)/Gamemode(s) instead of Lua S&box uses C#.
S&box allows its users to use C# to make complicated stuff(like procedural worlds)without touching C++.
To do: :add more information.