Actually there's some sort of error in everyone's thinking here. Accuracy of moves DOES NOT increase under Gravity. However, Evasion does decrease, by -2 stages. Effectively it does make less accurate moves hit more often, but just be sure to clarify the real effect.
I'm not sure this was ever tested, but I believe that Gravity's accuracy/evasion effects are not added or subtracted to the 'stages' of evasion and accuracy, but rather are an overlaying multiplier. Just as Tailwind doubles Speed rather than adding two stages, I believe Gravity simply multiplies the accuracy of moves by 5/3 (≈66.6%). Whether this is 'actually' a decrease in evasion or an increase in accuracy isn't verifiable, nor does it really matter.

