Will S&box kill gmod?

It will definitely kill off the smaller servers and the smaller gamemodes. I think larger servers like Superior Servers and Moat.gg will probably stay afloat if they don't switch over to S&box immediately, but Garry's Mod will definitely take a large hit. You won't see custom game modes/servers anymore. It will only be TTT, Prop Hunt, DarkRP, Deathrun, etc. and you'll only see a select few for each, not the plethora of servers you see now.