Will S&box kill gmod?

Well it depends. Facepunch is not planning on making a gmod 2 and I doubt S&box will have a good workshop system which gmod heavily relies on.

Yes! S&Box will be an improved version of GMOD. People will start playing S & Box, and GMOD will be abandoned.

Did you see HL Alyx gameplay? It's a revolution, lol. Of course SBox will kill Gmod

Sbox promises to be optimized, which is why I'm also sure that it will kill Garry's mod