I'm unconvinced that Hyroen's points have been fully answered.
The problem here is that we can put any Gravity card in here. So, why submit cards for this "Challenge" when we can just submit them into the Gravity Crucible? On the other hand, why should the average card idea maker, who gets around to making card ideas for Gravity, want to submit their card into the Gravity Crucible when they can gain reputation/prestige by submitting it for this "Challenge"?
Basically, why is this thread/challenge even on the forums at all? Why is this named a "Challenge" when making a card that fulfills the requirements isn't challenging at all?