s&box as I understand will be a lot of things, including possibility to be a full game making platform with Source 2 as base engine. Much like Garry's Mod is able to have many different things done to it simply through LUA code and underlying source engine and garry's mod base code can handle networking and such without the developer needing to get a PHD.
S&box as I understand takes this aspect further with actual editors for all sorts of game development stuff, and worked on exporting the created games so people do not have to buy S&box, the creator does.
But it seems there is still possibility for good old Garry's Mod type of fun to do, as far as I've seen it is also just Garry's Mod Next Gen as well, if you wish to do that with it. Living up to it's name. So calling it "A game creation platform" is bit off kilter. It is that, but it is also a game, and other stuff.