What she said.
The way I understand it, is that Gravity Pull only effects one creature at a time. So if you have 6 Armaggios in play at once, all with Gravity Pull activated, it does not give you a theoretical hitpoint total of 250 = 100+25*6.
I think it would be a logistical coding nightmare of epic proportions to make it so that all incoming damage is spread equally between every creature which has the Gravity Pull buff. But, then again, it would be hellishly cool... *imagines a card which puts Gravity Pull on all enemy creatures in play for 3 turns*...
I might be wrong about the coding nightmare - I really know nothing at all when it comes to syntax and well.. ehh, yeah...