What is the best A passive for a bow hero?

Fury so that it helps develop into both his stats and vantage

I would recommend Fury more than L&D for Gaius since his resistance is good for suppressing magic within the current meta.

Though if you wanna do the typical bread and butter set of the Quad Strike (Brave Bow, L&D, Desperation, Threaten Defense), he has the stats to run it.

His Candy Bow stacks nicely with Fury.

I would pick my a-skill like this, swift sparrow 2 (cough cough) > life and death 3 ~= Death blow 3(if +spd ideally) > Fury 3.
It also depends on your weapon, if you use is original bow, swift sparrow 2 is best probably followed by Fury 3.