。
She was wearing a satin gown.
Literal
She [topic-は] satin [genitive-の] gown [object-を] was-wearing.
繻子 (しゅす) is an uncommon, slightly literary word for satin — most modern Japanese would simply use the loanword サテン. The kanji 繻 ('silk fabric') doesn't appear in everyday vocabulary, and an alternative spelling 朱子 covers the same word. ガウン is a loanword for a long, loose-fitting outer garment — anything from a dressing gown to a hospital gown to an academic gown, depending on context. The combination 繻子のガウン evokes formal or vintage attire, the kind of wording you'd find in older literature rather than a modern fashion blurb. Note 着ていた is the resultant-state continuous in the past, just as in everyday 'she was wearing.'