Gravity won't increase the velocity of a shot fired downwards.
The pellet leaves the muzzle with an initial velocity. It is slowed by wind resistance regardless of the direction up or down. Now the uphill shot is also subjected to a slowing force due to gravity. It follows that the downhill shot is not slowed by gravity,but the acceleration due to gravity works against the deceleration due to wind resistance.
As wind resistance is hugely dominant, you can safely ignore any downward acceleration of the pellet.