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)
Yes, it is. It's worked like that since the status was invented as far as I know. Why? Because it makes sense. Gravity Pull is both a defensive measure and the single most powerful way of destroying one creature. Imagine two or more creatures having gravity pull. Either the enemy's creatures would be hovering between the two, making them useless, or the gravity pull would be too weak to stop anything, and the creatures would attack the elemental.
I don't think it has anything to do with coding. It'd be easy to add another target, but that's not how the card is supposed to work. Mainly because it would be a bit too powerful as CC if it did.
Yes, he could change the GP target if he needed to. Costs him a card, and would rarely be a scenario where it matters. Also never seen CPU do that. I think they're pleased as long as one creature on your board is GP'd.
On the flip side, I actually find it useful when it comes to defence. It means you can switch between Armagios without killing one of them when it's low on HP. With Guardian Angels/Archangels, you can then heal the one that's lower on health. Had both kept the GP status, the one with lower HP would have been lost forever.