I would say that it is slowly dying but there will be people still playing but another thing is it might not die due to all the mods being made for the game and still getting supported by the community.
This question gets asked a lot, and I answer it like this every single time. Garry's Mod is not dying it's just a phase right now, especially as an old game it will not be as popular as it was back in GMod 11 & 12. Games, in general, die because they become the same repetitive thing that players are used to. Only thing to hope to keep the thriving Sandbox community to enjoy Facepunch's games is waiting for S&box which is being developed as we speak.
Not at all, he will live forever! On the Russian-speaking servers everything is in order and no one even thinks about the death of Gmod. Gmod is a great game of generations and its life will never end.