Does anyone know how to convert GPP(grains of moisture per pound of dry air) to G/KG(grains per KG)
I think you really want grams per kilogram.
7000 grains = 1 lb, so first divide by 7000 to convert the grains in the numerator to pounds. Now you have a small dimensionless number (pounds over pounds) which would be the same in kilograms/kilograms. 1 kg = 1000 g, so multiply by 1000 to convert the numerator to grams.
The net of these two operations is to divide by 7.