GC needs to fit specific conditions to be called a GC for a season.
It needs to be a very high up anime that is very popular front page anime (one of those that you know are front page kind before airing) with a very high budget, grade A voice actors all throughout, Grade A animation and that's about it.
From the looks of Shinsekai yori, it only faked it. The promo poster was done professoinaly with "those" type of realistic and soft clannad type of style and colors for the trailer and teaser. The info made it seem like it'd be very big and very ambitious. It was but just not done well. Next, it doens't have a high budget. It only faked it to seem so. It only has half the main characters being grade A so there is that. And an excellent ED. So while it fits half of the conditions, it doesn't fit 100% the conditions for a GC.