The Snowy pattern is wildtype mallard plus a recessive gene, so a Gray pair certainly could produce that if both are carrying that recessive gene. 
I honestly don't know how color leaking works with ducks as I have no experience there, but spitballing off what I know of color leakage in chickens... Extended black, E, is the gene that causes solid color all over in ducks, dominant to the wildtype, e+, that is the mallard pattern. Maybe a duck that is E/e+ split, perhaps with other genes at play as well, could have enough color leakage to look similar to an e+/e+ duck rather than being solid-colored? Pure speculation on my part! But something similar can happen in chickens. For example, when you cross a Buff Orpington to a Black Australorp in chickens, the solid black coloring of the Australorp (also caused by extended black, E, in chickens) is the dominant trait, but the offspring get so much color leakage from the Buff side of the cross that they often end up looking more Buff than Black.

I honestly don't know how color leaking works with ducks as I have no experience there, but spitballing off what I know of color leakage in chickens... Extended black, E, is the gene that causes solid color all over in ducks, dominant to the wildtype, e+, that is the mallard pattern. Maybe a duck that is E/e+ split, perhaps with other genes at play as well, could have enough color leakage to look similar to an e+/e+ duck rather than being solid-colored? Pure speculation on my part! But something similar can happen in chickens. For example, when you cross a Buff Orpington to a Black Australorp in chickens, the solid black coloring of the Australorp (also caused by extended black, E, in chickens) is the dominant trait, but the offspring get so much color leakage from the Buff side of the cross that they often end up looking more Buff than Black.