That's because Gravity Pull can only be inflicted on one creature per side at a time.
Is that a game rule? Since when? Why? Whatever.
This makes Gravity Pull waaaaay less useful as a way to protect yourself, then.
Imagine having 2x Armagios, and an opponent with 50 total damage from creatures, and you having 15 life... Thanks to this stupid rule, you'll die even if you could otherwise handle the damage for a turn which might be enough to win.
Yes, it's easier to code this than a way to distribute damage amongst multiple creatures with gravity pull properly, but come on! Game rules shouldn't be decided based on what is easier or harder to code, it should be decided based on how it plays better and how it is easier to understand and more logical.
Besides, with this kind of behavior, if my opponent plays a Gravity pull of his own on one of my small creatures, he can bypass my armagio/doll whatever I want to take the damage, and kill me. (Not with this deck, but in one that has other creatures he could)