Gala garb (4)
(Other definitions for gown that I've seen before include "Shift" , "Garment (academic, formal, or protective)" , "Elegant dress" , "Formal robe" , "Long formal dress" .)
'gala garb' is the definition.
(I've seen this in another clue)
This is all the clue.
(Another definition for tux that I've seen is " Briefly, American dinnerjacket".)