When you dream of your parent in a white gown?

There is not enough information to provide a meaningful interpretation. The "parent" might be the father or mother, and a "gown" could be a wedding gown, hospital gown, graduation gown, nightgown, evening gown, etc. Seeing one's father in an evening gown would mean something very different from seeing one's mother in the same dress, and a hospital gown would be very different from a graduation gown.