By definition, the sea level has a constant gravity.
Well that's not really true, see latitude and density effects.
Anyhow the model seems designed to deal with items which start high up at over 30,000 ft and end low down, ergo they will see a variation in gravitational pull. Now I think that this is likely to make little or no difference compared to wind effects varying with altitude, but I don't know.
Surely the beauty of a model which accounts for varying gravity is that you can find out. If after enough runs of the model to account for statistical significance it turns out that it is irrelevant then you can remove it. If it does make a difference leave it in.