It didn't. Gravity was originally AST's version of Holy (since it's original kit sans cards was cribbed directly from WHM). At one point in Heavensward, they had the same base potency but the main difference (apart from the obvious ranged target vs point blank range) was that Gravity had a lower cast time (Holy used to be a 3s cast) and Holy had the stun. Both have undergone a few changes since then.