It's like comparing apples to oranges,
however Garry's Mod will probably have an infinite lifetime because of it being powered by community addons (some of which I've done myself!),
while with Team Fortress 2, it has limited modding capabilities due to the closed nature of its code without any possibility to extend
(except for server sourcemod pluginsand even then there's only so much you can do with it)
and content would have to be added by Valve & TF Team themselves and I'm pretty sure TF2 has like, 8-11 guys working on it (not a lot).