Is it possible to have an idea of how amny copies of a clickbank product have been sold by its Gravity number?
Nope. One affiliate can make you 100 sales a day and it will still only show 1 gravity point. You can have 30 gravity and be making 10 sales a day by fluctuating aff's, and everything in between.. You can assume/guesstimate, but it's still a shot in the dark.
+1 , What he said is correct. I must add, that most of the time, the higher the gravity of the product, the more sales they make. You can't really expect a product with 10 gravity to make as much sales as a product with 100 gravity.