Mog, you are right in principle, the downward shot picks up a downward component due to gravity accelerating the pellet.
However the acceleration due to gravity is tiny compared to the deceleration due to the wind resistance or drag, by a factor of something like 50 to 100 times, so you can safely ignore it for airguns.
I find the way to follow it is to remember that the pellet falls in the vertical plane the same amount (at a given distance) whether your rifle is level or shooting uphill or downhill. The difference is that a horizontal shot lets you see that drop, broadside, whereas with a shot up or down hill you are looking at that same distance but from an angle, so it doesn't look so big. Imagine a normal target card, it's about 17cm square. Now look at it tilted at 45 degrees, your brain knows it's still 17cm square but it "looks" half the height.