Никогда не встречал gown в смысле просто "платье"

and what does "in a soaked dress/gown" even mean?

SOAK is when you fill a basin with a liquid and let the garment sit in it for a certain period of time - to remove stains, to dye it, etc.

замочить/замачивать - по-русски

So is the Russian here describing a dress that someone was wearing in the rain, perhaps?

Yes, (soaking) wet, I think

absolutely

Промокшее насквозь

Russ, your translation makes no sense