Gravity only affects the horizontal distance traveled.

For example, assume on level ground that the distance from the base of your tree is 30 yards to your target and that the actual (diagonal)distance from the treestand to the target is 32 yards.

Gravity would only act on your arrow as if it had traveled 30 yards, even though it had actually traveled 32 yards. You should therefore aim as you would normally on a 30 yard shot.