Ah, I have found my answer. This gravity spell is similar to the Atomos eidelon in FFIX, it can do 100% damage if you have enough of something. In this case, it's MP.

Gravity = ((MP x 5) + 40)% Damage
Gravira = ((MP x 5) + 55)% Damage
Gravija = ((MP X 5) + 70)% Damage

So, Gravira can do 100% Damage if Sora has an MP of 9. Of course this number varies depending on the type of heartless you're fighting. And after you seal the Hollow Bastion, the heartless become more resistant to Gravity, so you have to have more MP to have the same result. So I guess Donald having 14 MP could kill most anything with a Gravija spell.