I've a Hoya Gradual-Color gray filter.
May I know whats the purpose of it ?

Issit to tone the sky (from over exposure) as there is a greyish tint on 1 half of the filter ?

And is this Gradual functions as ND filter ? ( a bit confused) coz the seller told me its an ND filter initially. So I was wondering if it can acheive those nice silky waterfall effect.