a cocktail gown is 'une robe de cocktail' in French
A wedding gown is 'une robe de mariée' in French.
The word sought may be negligee (nightie, dressing gown) from the French (négligé).(The similar word is negligence, culpable neglect.)
Yes, that is one meaning of the word 'gown'.
The Japanese word Wedding Gown is ウェディングドレス.
une robe (fem.)
She donned a gown of flowing silk.
Gown is one syllable, so say it just like it looks.
une chemise de nuit
It's "Robe de Bal".
Fedora, French Gown
It has 1 syllable.