Alright so this is something a lot of people are talking about recently and it seems like the answers are pretty much in for it. So we figured we would make a post to answer it once and for all based on the information we currently have.
S&box is going to naively support addon creation in C# – while Garry’s Mod uses Lua. It’s unlikely Garry’s Mod addons will work in S&box out of the box. However this doesn’t mean that all hope is lost, because there is some interesting stuff on the maps, models, materials & sounds front.
Facepunch have made it pretty clear their goal is to – like UE4 – support many different file formats for maps, models, textures & sounds – without you needing to do anything. It’ll just work like magic. Source
This is a real advantage, because currently if people want to port models or maps from other games in Garry’s Mod, they typically need to put some work in to make the assets work. With this new system it’ll be pretty much plug and play – this is a big deal. This means the library of assets people can use natively in Sandbox will be exponentially larger.
So the answer to the initial question – “Will Garry’s Mod addons, models and maps work in sbox?” – is no, and yes. It kinda depends on what it is that you’re hoping to use in Sbox.
Looking for addons for your server? Check out Crident Store: https://crident.store