I actually don't play garry's mod that much anymore, When I'm playing it I'm normally coding Lua or just messing about with Leaks really - I think the game its self wont die for a while but I'm more hyped for S&Box then actual Garry's Mod at this point, Maybe if Garry's Mod got a new engine and went straight to 64 bit I'd play it but this is un-likely
I have like 600 hours on the game and less than 10 have been on Multiplayer. If you know what to do on Singleplayer Sandbox you completely forget the game can even be played with others.