Ok, well I read over the GPL again and went over the Wikipedia article on it.
My understanding of it is that you WOULD need to GPL release Project C, not because it’s a derivitive of Project A, rather because Project B is forcibly GPL-released, any derivitive of that work (which is Project B) would need to be released (which would mean Project C). As far as I know, the standpoint legally is that the difference between Projects A and B is not unlike the difference between Projects B and C, meaning Project C would need to be released.
Domino effect, baby!