Will Garry's Mod Die?!?

Unfortunately. Yes. At some point, Gmod will die.
Just like every game in existence. But, I think it will take a very long time.

Due to the simple fact, Nothing to replace it was made yet.
You cannot play DarkRP or TTT in any other game to the same extent.
And Gmod modability is still amazing.
Until something comes out which can take over Gmod throne. I don't see Gmod going away completely.